EX-4.2





 Exhibit 4.2 

[FORM OF CLASS B WARRANT] 

X4 PHARMACEUTICALS, INC. 

CLASS B WARRANT TO PURCHASE COMMON STOCK 

Warrant No.: 
 Number of Shares of Common Stock: 

Date of Issuance: November 29, 2019 (“Issuance Date”) 

X4 Pharmaceuticals, Inc., a Delaware corporation (the “Company”), hereby certifies that, for good and valuable consideration, the receipt and
sufficiency of which are hereby acknowledged, , the registered holder hereof or its permitted assigns (the “Holder”), is entitled, subject to the terms set forth below, to purchase from the Company, at the Exercise Price (as defined
below) then in effect, at any time or times on or after the Issuance Date (the “Initial Exercisability Date”), but not after 11:59 p.m., New York time, on the Expiration Date (as defined below),
([                    ] ) fully paid non-assessable shares of Common Stock (as defined below), subject
to adjustment as provided herein (the “Warrant Shares”). Except as otherwise defined herein, capitalized terms in this Class B Warrant to Purchase Common Stock (including any Warrants to Purchase Common Stock issued in
exchange, transfer or replacement hereof, this “Warrant”), shall have the meanings set forth in Section 15. This Warrant is one of the Warrants to Purchase Common Stock (the “Warrants”) issued in connection
with the transactions contemplated by (i) that certain Underwriting Agreement, dated as of November 26, 2019 (the “Subscription Date”), by and between the Company and Cowen and Company, LLC and Stifel, Nicolaus &
Company, Incorporated, as the representatives of the several underwriters named therein (the “Underwriting Agreement”), (ii) the Company’s Registration Statement on Form S-3 (File number 333-233161) (the “Registration Statement”) and (iii) the Company’s prospectus supplement dated as of November 26, 2019 (the “Prospectus Supplement”). 

1. EXERCISE OF WARRANT. 
 (a) Mechanics of
Exercise. Subject to the terms and conditions hereof (including, without limitation, the limitations set forth in Section 1(f)), this Warrant may be exercised by the Holder at any time or times on or after the Initial Exercisability Date,
in whole or in part, by delivery (whether via facsimile, electronic mail or otherwise) of a written notice, in the form attached hereto as Exhibit A (the “Exercise Notice”), of the Holder’s election to exercise this
Warrant. Within one (1) Trading Day following the delivery of the Exercise Notice, the Holder shall make payment to the Company of an amount equal to the Exercise Price in effect on the date of such exercise multiplied by the number of Warrant
Shares as to which this Warrant is being exercised (the “Aggregate Exercise Price”) in cash by wire transfer of immediately available funds or, if the provisions of Section 1(d) are applicable, by notifying the Company that
this Warrant is being exercised pursuant to a Cashless Exercise (as defined below). The Holder shall not be required to deliver the original Warrant in order to effect an exercise hereunder (until the Holder has purchased all of the Warrant Shares
available hereunder and the Warrant has been exercised in full), nor shall any ink-original signature or medallion guarantee (or other type of guarantee or notarization) with respect to any Exercise Notice be
required. Execution and delivery of the Exercise Notice with respect to less than all of the Warrant Shares shall have the same effect as cancellation of the original Warrant and issuance of a new Warrant evidencing the right to purchase the
remaining number of Warrant Shares and the Holder shall not be required to physically surrender this Warrant to the Company until the Holder has purchased all of the Warrant Shares available hereunder and the Warrant has been exercised in full, in
which case, the Holder shall surrender this Warrant to the Company for cancellation within three (3) Trading Days of the date on which the final Exercise Notice is delivered to the Company. On or before the first (1st) Trading Day following the date on which the Holder has delivered the applicable Exercise Notice, the Company shall transmit by facsimile or electronic mail an acknowledgment of confirmation of
receipt of the Exercise Notice, in the form attached to the Exercise Notice, to the Holder and the Company’s transfer agent (the “Transfer Agent”). So long as the Holder delivers the Aggregate Exercise Price (or notice of a
Cashless Exercise, if applicable) on or prior to the first (1st) Trading Day following the date on which the Exercise Notice has been delivered to the Company, then on or prior to the earlier of
(i) the second (2nd) Trading Day and (ii) the number of Trading Days comprising the Standard Settlement Period, in each case following the date on which the Exercise Notice has been
delivered to the Company, or, if the Holder does not deliver the Aggregate Exercise Price (or notice of a Cashless Exercise, if applicable) on or prior to the first (1st) Trading Day following the
date on which the Exercise Notice has been delivered to the Company, then on or prior to the first (1st) Trading Day following the date on which the Aggregate Exercise Price (or notice of a
Cashless Exercise, if applicable) is delivered (such earlier date, or if later, the earliest day on which the Company is required to deliver Warrant Shares pursuant to this Section 1(a), the “Share Delivery Date”), the Company
shall (X) provided that the Transfer Agent is participating in The Depository Trust Company (“DTC”) Fast Automated Securities Transfer Program, credit such aggregate number of Warrant Shares to which the Holder is
entitled pursuant to such exercise to the Holder’s or its designee’s balance account with DTC 












 
through its Deposit / Withdrawal At Custodian system, or (Y) if the Transfer Agent is not participating in the DTC Fast Automated Securities Transfer Program (“FAST”), issue
and dispatch by overnight courier to the address as specified in the Exercise Notice, a certificate, registered in the name of the Holder or its designee, for the number of Warrant Shares to which the Holder is entitled pursuant to such exercise.
The Company shall be responsible for all fees and expenses of the Transfer Agent and all fees and expenses with respect to the issuance of Warrant Shares via DTC, if any, including without limitation for same day processing. Upon delivery of the
Exercise Notice, the Holder shall be deemed for all corporate purposes to have become the holder of record and beneficial owner of the Warrant Shares with respect to which this Warrant has been exercised, irrespective of the date such Warrant Shares
are credited to the Holder’s DTC account or the date of delivery of the certificates evidencing such Warrant Shares, as the case may be. If this Warrant is physically delivered to the Company in connection with any exercise pursuant to this
Section 1(a) and the number of Warrant Shares represented by this Warrant submitted for exercise is greater than the number of Warrant Shares being acquired upon an exercise, then the Company shall as soon as practicable and in no event later
than three (3) Trading Days after any exercise and at its own expense, issue and deliver to the Holder (or its designee) a new Warrant (in accordance with Section 6(d)) representing the right to purchase the number of Warrant Shares
issuable immediately prior to such exercise under this Warrant, less the number of Warrant Shares with respect to which this Warrant is exercised. No fractional Warrant Shares are to be issued upon the exercise of this Warrant, but rather the number
of Warrant Shares to be issued shall be rounded to the nearest whole number. The Company shall pay any and all transfer, stamp, issuance and similar taxes, costs and expenses (including, without limitation, fees and expenses of the Transfer Agent)
which may be payable with respect to the issuance and delivery of Warrant Shares upon exercise of this Warrant. The Company’s obligations to issue and deliver Warrant Shares in accordance with the terms and subject to the conditions hereof are
absolute and unconditional, irrespective of any action or inaction by the Holder to enforce the same, any waiver or consent with respect to any provision hereof, the recovery of any judgment against any Person or any action to enforce the same, or
any setoff, counterclaim, recoupment, limitation or termination; provided, however, that the Company shall not be required to deliver Warrant Shares with respect to an exercise prior to the Holder’s delivery of the Aggregate
Exercise Price (or notice of a Cashless Exercise, if applicable) with respect to such exercise. If the Holder is not permitted to exercise this Warrant for Warrant Shares due to Section 1(f) hereof, then the Holder may exercise this Warrant for
an equivalent number of Pre-Funded Warrants with an exercise price of $0.001 in substantially the same form of Pre-Funded Warrant to purchase shares of common stock
described in the Prospectus Supplement. 
 (b) Exercise Price. For purposes of this Warrant, “Exercise Price” means $15.00 per
share, subject to adjustment as provided herein. 
 (c) Company’s Failure to Timely Deliver Securities. In addition to any other rights
available to the Holder, if the Company fails to cause the Transfer Agent to transmit to the Holder the Warrant Shares in accordance with the provisions of Section 1(a) above pursuant to an exercise on or before the Share Delivery Date (other
than a failure caused by incorrect or incomplete information provided by the Holder to the Company), and if after such date the Holder is required by its broker to purchase (in an open market transaction or otherwise) or the Holder’s brokerage
firm otherwise purchases, shares of Common Stock to deliver in satisfaction of a sale by the Holder of the Warrant Shares which the Holder anticipated receiving upon such exercise (a
“Buy-In”), then the Company shall (A) pay in cash to the Holder the amount, if any, by which (x) the Holder’s total purchase price (including brokerage commissions, if any) for
the shares of Common Stock so purchased exceeds (y) the amount obtained by multiplying (1) the number of Warrant Shares that the Company was required to deliver to the Holder in connection with the exercise at issue times (2) the
price at which the sell order giving rise to such purchase obligation was executed, and (B) at the option of the Holder, either reinstate the portion of the Warrant and equivalent number of Warrant Shares for which such exercise was not honored
(in which case such exercise shall be deemed rescinded) or deliver to the Holder the number of shares of Common Stock that would have been issued had the Company timely complied with its exercise and delivery obligations hereunder. For example, if
the Holder purchases Common Stock having a total purchase price of $11,000 to cover a Buy-In with respect to an attempted exercise of shares of Common Stock with an aggregate sale price giving rise to such
purchase obligation of $10,000, under clause (A) of the immediately preceding sentence, the Company shall be required to pay the Holder $1,000. The Holder shall provide the Company written notice within three (3) Trading Days after the
occurrence of a Buy-In, indicating the amounts payable to the Holder in respect of the Buy-In and, upon request of the Company, evidence of the amount of such loss.
Nothing herein shall limit a Holder’s right to pursue any other remedies available to it hereunder, at law or in equity including, without limitation, a decree of specific performance and/or injunctive relief with respect to the Company’s
failure to timely deliver shares of Common Stock upon exercise of the Warrant as required pursuant to the terms hereof. As of the Issuance Date, the Company’s current transfer agent participates in FAST. In the event that the Company changes
transfer agents while this Warrant is outstanding, the Company shall use commercially reasonable efforts to select a transfer agent that participates in FAST. While this Warrant is outstanding, the Company shall request its transfer agent to
participate in FAST with respect to this Warrant. 
 (d) Cashless Exercise. If at the time of exercise hereof there is no effective registration
statement registering, or the prospectus contained therein is not available for, the issuance of the Warrant Shares to the Holder, then the Holder may exercise this Warrant in whole or in part and, in lieu of making the cash payment otherwise
contemplated to be made to the 












 
Company upon such exercise in payment of the Aggregate Exercise Price, elect instead to receive upon such exercise the “Net Number” of shares of Common Stock determined according to the
following formula (a “Cashless Exercise”): 
  

			
	 Net Number =
	  	 (A x B) - (A x C)

		  	B


 For purposes of the foregoing formula: 
  

			
	 A=
	  	the total number of shares with respect to which this Warrant is then being exercised.
		
	 B

=
	  	as applicable: (i) the Closing Sale Price of the Common Stock on the Trading Day immediately preceding the date of the applicable Exercise Notice if such Exercise Notice is (1) both executed and delivered pursuant to
Section 1(a) hereof on a day that is not a Trading Day or (2) both executed and delivered pursuant to Section 1(a) hereof on a Trading Day prior to the opening of “regular trading hours” (as defined in Rule 600(b)(64) of
Regulation NMS promulgated under the federal securities laws) on such Trading Day, (ii) at the option of the Holder, either (y) the Weighted Average Price on the Trading Day immediately preceding the date of the applicable Exercise Notice
or (z) the Bid Price of the Common Stock as of the time of the Holder’s execution of the applicable Exercise Notice if such Exercise Notice is executed during “regular trading hours” on a Trading Day and is delivered within two
(2) hours thereafter (including until two (2) hours after the close of “regular trading hours” on a Trading Day) pursuant to Section 1(a) hereof or (iii) the Closing Sale Price of the Common Stock on the date of the
applicable Exercise Notice if the date of such Exercise Notice is a Trading Day and such Exercise Notice is both executed and delivered pursuant to Section 1(a) hereof after the close of “regular trading hours” on such Trading
Day.
		
	 C=
	  	the Exercise Price then in effect for the applicable Warrant Shares at the time of such exercise.

 If Warrant Shares are issued in such a cashless exercise, the Company acknowledges and agrees that in accordance with
Section 3(a)(9) of the Securities Act of 1933, as amended, the Warrant Shares shall take on the registered characteristics of the Warrants being exercised, and the holding period of the Warrants being exercised may be tacked on to the holding
period of the Warrant Shares. The Company agrees not to take any position contrary to this Section 1(d). 
 (e) Disputes. In the case of a
dispute as to the determination of the Exercise Price or the arithmetic calculation of the Warrant Shares, the Company shall promptly issue to the Holder the number of Warrant Shares that are not disputed and resolve such dispute in accordance with
Section 10. 
 (f) Beneficial Ownership. Notwithstanding anything to the contrary contained herein, the Company shall not effect the exercise of
any portion of this Warrant, and the Holder shall not have the right to exercise any portion of this Warrant, pursuant to the terms and conditions of this Warrant and any such exercise shall be null and void and treated as if never made, to the
extent that immediately prior to or after giving effect to such exercise, the Holder together with the other Attribution Parties collectively would beneficially own in excess of 9.99% (the “Maximum Percentage”) of the number of
shares of Common Stock outstanding immediately after giving effect to such exercise. For purposes of the foregoing sentence, the aggregate number of shares of Common Stock beneficially owned by the Holder and the other Attribution Parties shall
include the number of shares of Common Stock held by the Holder and all other Attribution Parties plus the number of shares of Common Stock issuable upon exercise of this Warrant with respect to which the determination of such sentence is being
made, but shall exclude the number of shares of Common Stock which would be issuable upon (A) exercise of the remaining, unexercised portion of this Warrant beneficially owned by the Holder or any of the other Attribution Parties and
(B) exercise or conversion of the unexercised or unconverted portion of any other securities of the Company (including, without limitation, any convertible notes or convertible preferred stock or warrants, including the other Warrants)
beneficially owned by the Holder or any other Attribution Party subject to a limitation on conversion or exercise analogous to the limitation contained in this Section 1(f). For purposes of this Section 1(f), beneficial ownership shall be
calculated in accordance with Section 13(d) of the Securities Exchange Act of 1934, as amended (the “1934 Act”), it being acknowledged by the Holder that the Company is not representing to the Holder that such calculation is in
compliance with Section 13(d) of the 1934 Act and the Holder is solely responsible for any schedules required to be filed in accordance therewith. To the extent that the limitation contained in this Section 1(f) applies, the determination
of whether this Warrant is exercisable (in relation to other securities owned by the Holder together with any Affiliates and Attribution Parties) and of which portion of this Warrant is exercisable shall be in the sole discretion of the Holder, and
the Company shall have no obligation to verify or confirm the accuracy of such determination. In addition, a determination as to any group status as contemplated above shall be determined in accordance with Section 13(d) of the 1934 Act and the
rules and regulations promulgated thereunder. For purposes of this Warrant, in determining the number of outstanding shares of Common Stock the Holder may acquire upon the exercise of this Warrant without exceeding the Maximum Percentage, the Holder
may rely on the number of outstanding shares of Common Stock as reflected in (x) the Company’s most recent Annual Report on Form 10-K, Quarterly Report on Form
10-Q and Current Reports on Form 8-K or other public filing with the Securities and Exchange Commission (the 












 
“SEC”), as the case may be, (y) a more recent public announcement by the Company or (z) any other written notice by the Company setting forth the number of shares of
Common Stock outstanding (the “Reported Outstanding Share Number”). If the Company receives an Exercise Notice from the Holder at a time when the actual number of outstanding shares of Common Stock is less than the Reported
Outstanding Share Number, the Company shall (i) notify the Holder in writing of the number of shares of Common Stock then outstanding and, to the extent that such Exercise Notice would otherwise cause the Holder’s beneficial ownership, as
determined pursuant to this Section 1(f), to exceed the Maximum Percentage, the Holder must notify the Company of a reduced number of Warrant Shares to be purchased pursuant to such Exercise Notice (the number of shares by which such purchase
is reduced, the “Reduction Shares”) and (ii) as soon as reasonably practicable, the Company shall return to the Holder any exercise price paid by the Holder for the Reduction Shares. For any reason at any time, upon the written
or oral request of the Holder, the Company shall within one (1) Business Day confirm orally and in writing or by electronic mail to the Holder the number of shares of Common Stock then outstanding. In any case, the number of outstanding shares
of Common Stock shall be determined after giving effect to the conversion or exercise of securities of the Company, including this Warrant, by the Holder and any other Attribution Party since the date as of which the Reported Outstanding Share
Number was reported. In the event that the issuance of Common Stock to the Holder upon exercise of this Warrant results in the Holder and the other Attribution Parties being deemed to beneficially own, in the aggregate, more than the Maximum
Percentage of the number of outstanding shares of Common Stock (as determined under Section 13(d) of the 1934 Act), the number of shares so issued by which the Holder’s and the other Attribution Parties’ aggregate beneficial ownership
exceeds the Maximum Percentage (the “Excess Shares”) shall be deemed null and void and shall be cancelled ab initio, and the Holder shall not have the power to vote or to transfer the Excess Shares. As soon as reasonably practicable
after the issuance of the Excess Shares has been deemed null and void, the Company shall return to the Holder the exercise price paid by the Holder for the Excess Shares. Upon delivery of a written notice to the Company, the Holder may from time to
time increase or decrease the Maximum Percentage to any other percentage (not in excess of 19.99% of the issued and outstanding shares of Common Stock immediately after giving effect to the issuance of the shares of Common Stock issuable upon
exercise of this Warrant if exceeding that limit would result in a change of control under Nasdaq Listing Rule 5636(b) or any successor rule) as specified in such notice; provided that (i) any such increase in the Maximum Percentage will
not be effective until the sixty-first (61st) day after such notice is delivered to the Company and (ii) any such increase or decrease will apply only to the Holder and the other Attribution
Parties and not to any other holder of Warrants that is not an Attribution Party of the Holder. For purposes of clarity, the shares of Common Stock issuable pursuant to the terms of this Warrant in excess of the Maximum Percentage shall not be
deemed to be beneficially owned by the Holder for any purpose including for purposes of Section 13(d) or Rule 16a-1(a)(1) of the 1934 Act. No prior inability to exercise this Warrant pursuant to this
paragraph shall have any effect on the applicability of the provisions of this paragraph with respect to any subsequent determination of exercisability. The provisions of this paragraph shall be construed and implemented in a manner otherwise than
in strict conformity with the terms of this Section 1(f) to the extent necessary to correct this paragraph or any portion of this paragraph which may be defective or inconsistent with the intended beneficial ownership limitation contained in
this Section 1(f) or to make changes or supplements necessary or desirable to properly give effect to such limitation. The limitation contained in this paragraph may not be waived and shall apply to a successor holder of this Warrant. 

(g) Required Reserve Amount. So long as this Warrant remains outstanding, the Company shall at all times keep reserved for issuance under this Warrant
a number of shares of Common Stock at least equal to 100% of the maximum number of shares of Common Stock as shall be necessary to satisfy the Company’s obligation to issue shares of Common Stock under the Warrants then outstanding (without
regard to any limitations on exercise) (the “Required Reserve Amount”); provided that at no time shall the number of shares of Common Stock reserved pursuant to this Section 1(g) be reduced other than in connection with
any exercise of Warrants or such other event covered by Section 2 below. The Required Reserve Amount (including, without limitation, each increase in the number of shares so reserved) shall be allocated pro rata among the holders of the
Warrants based on the number of shares of Common Stock issuable upon exercise of Warrants held by each holder thereof on the Issuance Date (without regard to any limitations on exercise) (the “Authorized Share Allocation”). In the
event that a holder shall sell or otherwise transfer any of such holder’s Warrants, each transferee shall be allocated a pro rata portion of such holder’s Authorized Share Allocation. Any shares of Common Stock reserved and allocated to
any Person which ceases to hold any Warrants shall be allocated to the remaining holders of Warrants, pro rata based on the number of shares of Common Stock issuable upon exercise of the Warrants then held by such holders thereof (without regard to
any limitations on exercise). 
 (h) Insufficient Authorized Shares. Without limiting or waiving any of the other rights the Holder may have under
law, if at any time while this Warrant remains outstanding the Company does not have a sufficient number of authorized and unreserved shares of Common Stock to satisfy its obligation to reserve for issuance the Required Reserve Amount (an
“Authorized Share Failure”), then the Company shall promptly take all commercially reasonable actions necessary to increase the Company’s authorized shares of Common Stock to an amount sufficient to allow the Company to reserve
the Required Reserve Amount for this Warrant then outstanding. Without limiting the generality of the foregoing sentence, as 












 
soon as practicable after the date of the occurrence of an Authorized Share Failure, but in no event later than ninety (90) days after the occurrence of such Authorized Share Failure, the
Company shall hold a meeting of its stockholders for the approval of an increase in the number of authorized shares of Common Stock. In connection with such meeting, the Company shall provide each stockholder with a proxy statement and shall use its
commercially reasonable efforts to solicit its stockholders’ approval of such increase in authorized shares of Common Stock and to cause its board of directors to recommend to the stockholders that they approve such proposal. Notwithstanding
the foregoing, if any such time of an Authorized Share Failure, the Company is able to obtain the written consent of a majority of the shares of its issued and outstanding shares of Common Stock to approve the increase in the number of authorized
shares of Common Stock, the Company may satisfy this obligation by obtaining such consent and submitting for filing with the SEC an Information Statement on Schedule 14C. 

2. ADJUSTMENT OF EXERCISE PRICE AND NUMBER OF WARRANT SHARES UPON CERTAIN EVENTS. 

(a) One-Time Exercise Price Adjustment. If, prior to the Expiration Date, the Company sells shares of Common
Stock or derivative securities convertible into or exercisable for Common Stock (other than Exempted Securities) in one or more related transactions primarily for the purpose of raising capital (each, a “Subsequent Offering”) at an
Offering Weighted-Average Price below $15.00 per share (as may be adjusted by paragraphs (b) through (e)), then the initial Exercise Price of this Warrant will be automatically reset upon exercise of this Warrant to an exercise price (the
“Adjusted Exercise Price”) that is the lowest Offering Weighted-Average Price per share at which the Company sells Common Stock or derivative securities convertible into or exercisable for Common Stock in a Subsequent Offering prior
to the exercise date; provided, however, that the Adjusted Exercise Price will not be reduced below $1.50 per share. For purposes of this Section 2(a), the “Offering Weighted-Average Price” shall be calculated as the
weighted-average Common Stock equivalent price of the equity securities sold in such transaction(s) (excluding any derivative securities with an exercise or conversion price that is above the Closing Sale Price as of the time of pricing such
offering(s)), and subtracting the value of any warrants or other derivative securities sold in such offering (determining the value of such derivative securities in accordance with GAAP using the Black-Scholes Option Pricing Model and dividing the
warrant or derivative value by the number of shares potentially issuable thereunder). For example, if the Company were to conduct a capital raising transaction where it sold 100 shares of Common Stock at $8.00 per share, but also provided 25%
warrant coverage with an exercise price of $6.00 per share and a per-share Black Scholes warrant value of $0.50, then the weighted average common stock equivalent price would be $7.20 per share (i.e., $900 in
total potential proceeds, divided by 125 total potential shares), and the Adjusted Exercise Price would equal $7.20 per share (rounded to the nearest cent). “Exempted Securities” means (a) shares of Common Stock or rights, warrants or
options to purchase Common Stock issued in connection with any acquisition of another corporation by the Company by merger, purchase of substantially all of the assets or other reorganization or to a joint venture agreement, provided that such
issuances are approved by the Board of Directors, (b) equity securities issued by reason of a dividend, stock split, split-up or other distribution on shares of Common Stock, (c) shares of Common
Stock or rights, warrants or options to purchase Common Stock issued to employees or directors of, or consultants or advisors to, the Company or any of its subsidiaries pursuant to a plan, agreement or arrangement approved by the Board of Directors,
(d) shares of Common Stock issued upon the exercise of options or upon the conversion or exchange of any securities convertible into Common Stock, including any pre-funded warrants or Class A
warrants outstanding as of the Issuance Date and any pre-funded warrants to be issued pursuant to the Underwriting Agreement, in each case provided that such issuance is pursuant to the terms of the applicable
option or convertible security, (e) shares of Common Stock or rights, warrants or options to purchase Common Stock issued to banks, equipment lessors or other financial institutions, real property lessors, or to other persons engaged in the
business of making loans, pursuant to a debt financing (including any refinancing or interest deferral or similar transaction related to existing debt outstanding as of the Issuance Date), equipment leasing or real property leasing transaction
approved by the Board of Directors, or (f) shares of Common Stock or rights, warrants or options to purchase Common Stock issued in connection with sponsored research, collaboration, technology license, development, marketing or other similar
agreements or strategic partnerships approved by the Board of Directors. 
 (b) If the Company at any time on or after the Subscription Date subdivides (by
any stock split, stock dividend, recapitalization or otherwise) one or more classes of its outstanding shares of Common Stock into a greater number of shares, the Exercise Price in effect immediately prior to such subdivision will be proportionately
reduced and the number of Warrant Shares will be proportionately increased. If the Company at any time on or after the Subscription Date combines (by combination, reverse stock split or otherwise) one or more classes of its outstanding shares of
Common Stock into a smaller number of shares, the Exercise Price in effect immediately prior to such combination will be proportionately increased and the number of Warrant Shares will be proportionately decreased. Any adjustment under this
Section 2(b) shall become effective at the close of business on the date the subdivision or combination becomes effective. 
 (c) Voluntary
Adjustment by Company. The Company may at any time during the term of this Warrant reduce the then current Exercise Price to any amount and for any period of time deemed appropriate by the Board of Directors of the Company. 












 (d) Rights Upon Distribution of Assets. In addition to any adjustments pursuant to the other
subsections of Section 2 above, if, on or after the Subscription Date and on or prior to the Expiration Date, the Company shall declare or make any dividend or other distribution of its assets (or rights to acquire its assets) to holders of
shares of Common Stock, by way of return of capital or otherwise (including, without limitation, any distribution of cash, stock or other securities, property, options, evidence of indebtedness or any other assets by way of a dividend, spin off,
reclassification, corporate rearrangement, scheme of arrangement or other similar transaction) (a “Distribution”), at any time after the issuance of this Warrant, then, in each such case, the Holder shall be entitled to participate
in such Distribution to the same extent that the Holder would have participated therein if the Holder had held the number of shares of Common Stock acquirable upon complete exercise of this Warrant (without regard to any limitations or restrictions
on exercise of this Warrant, including without limitation, the Maximum Percentage) immediately before the date on which a record is taken for such Distribution, or, if no such record is taken, the date as of which the record holders of shares of
Common Stock are to be determined for the participation in such Distribution (provided, that to the extent that the Holder’s right to participate in any such Distribution would result in the Holder and the other Attribution Parties
exceeding the Maximum Percentage, then the Holder shall not be entitled to participate in such Distribution to such extent (and shall not be entitled to beneficial ownership of such shares of Common Stock as a result of such Distribution (and
beneficial ownership) to such extent) and the portion of such Distribution shall be held in abeyance for the benefit of the Holder until such time or times as its right thereto would not result in the Holder and the other Attribution Parties
exceeding the Maximum Percentage, at which time or times the Holder shall be granted such Distribution (and any Distributions declared or made on such initial Distribution or on any subsequent Distribution held similarly in abeyance) to the same
extent as if there had been no such limitation). 
 (e) Purchase Rights. In addition to any adjustments pursuant to the other subsections of
Section 2 above, if at any time on or after the Subscription Date and on or prior to the Expiration Date the Company grants, issues or sells any Options, Convertible Securities or rights to purchase stock, warrants, securities or other
property, in each case pro rata to the record holders of any class of Common Stock (the “Purchase Rights”), then the Holder will be entitled to acquire, upon the terms applicable to such Purchase Rights, the aggregate Purchase
Rights which the Holder could have acquired if the Holder had held the number of shares of Common Stock acquirable upon complete exercise of this Warrant (without regard to any limitations or restrictions on exercise of this Warrant, including
without limitation, the Maximum Percentage) immediately before the date on which a record is taken for the grant, issuance or sale of such Purchase Rights, or, if no such record is taken, the date as of which the record holders of Common Stock are
to be determined for the grant, issuance or sale of such Purchase Rights (provided, that to the extent that the Holder’s right to participate in any such Purchase Right would result in the Holder and the other Attribution Parties
exceeding the Maximum Percentage, then the Holder shall not be entitled to participate in such Purchase Right to such extent (and shall not be entitled to beneficial ownership of such Common Stock as a result of such Purchase Right (and beneficial
ownership) to such extent) and such Purchase Right to such extent shall be held in abeyance for the benefit of the Holder until such time or times as its right thereto would not result in the Holder and the other Attribution Parties exceeding the
Maximum Percentage, at which time or times the Holder shall be granted such right (and any Purchase Right granted, issued or sold on such initial Purchase Right or on any subsequent Purchase Right to be held similarly in abeyance) to the same extent
as if there had been no such limitation). 
 3. FUNDAMENTAL TRANSACTIONS. The Company shall not enter into or be party to a Fundamental Transaction
unless the Successor Entity assumes in writing all of the obligations of the Company under this Warrant in accordance with the provisions of this Section 3, including agreements to deliver to the Holder in exchange for this Warrant a security
of the Successor Entity evidenced by a written instrument substantially similar in form and substance to this Warrant, including, without limitation, which is exercisable for a corresponding number of shares of capital stock equivalent to the shares
of Common Stock acquirable and receivable upon exercise of this Warrant (without regard to any limitations on the exercise of this Warrant) prior to such Fundamental Transaction, and with an exercise price which applies the exercise price hereunder
to such shares of capital stock (but taking into account the relative value of the shares of Common Stock pursuant to such Fundamental Transaction and the value of such shares of capital stock, such adjustments to the number of shares of capital
stock and such exercise price being for the purpose of protecting the economic value of this Warrant immediately prior to the consummation of such Fundamental Transaction). Upon the consummation of each Fundamental Transaction, the Successor Entity
shall succeed to, and be substituted for the Company (so that from and after the date of the applicable Fundamental Transaction, the provisions of this Warrant referring to the “Company” shall refer instead to the Successor Entity), and
may exercise every right and power of the Company and shall assume all of the obligations of the Company under this Warrant with the same effect as if such Successor Entity had been named as the Company herein. Notwithstanding the foregoing, and
without limiting Section 1(f) hereof, the Holder may elect, at its sole option, by delivery of written notice to the Company to waive this Section 3 to permit the Fundamental Transaction without the assumption of this Warrant. In addition
to and not in substitution for any other rights hereunder, prior to the consummation of each Fundamental Transaction pursuant to which holders of shares of Common Stock are entitled to receive securities or other assets with respect to or in
exchange for shares of Common Stock (a “Corporate Event”), the Company shall make appropriate provision to ensure that the Holder will thereafter have the right to receive upon an exercise of this Warrant at any time after the
consummation of the applicable 












 
Fundamental Transaction but prior to the Expiration Date, in lieu of the shares of the Common Stock (or other securities, cash, assets or other property) issuable upon the exercise of the Warrant
prior to such Fundamental Transaction, such shares of stock, securities, cash, assets or any other property whatsoever (including warrants or other purchase or subscription rights) (collectively, the “Corporate Event Consideration”)
which the Holder would have been entitled to receive upon the happening of the applicable Fundamental Transaction had this Warrant been exercised immediately prior to the applicable Fundamental Transaction (without regard to any limitations on the
exercise of this Warrant). The provision made pursuant to the preceding sentence shall be in a form and substance reasonably satisfactory to the Holder. The provisions of this Section 3 shall apply similarly and equally to successive
Fundamental Transactions and Corporate Events. Notwithstanding the foregoing, in the event of a Change of Control (other than a Change of Control which was not approved by the Board of Directors of the Company, as to which this right shall not
apply), at the request of the Holder delivered before the 30th day after such Change of Control, the Company (or the Successor Entity) shall purchase this Warrant from the Holder by paying to the Holder, within five (5) Business Days after such
request (or, if later, on the effective date of the Change of Control), an amount equal to the Black Scholes Value of the remaining unexercised portion of this Warrant on the effective date of such Change of Control, payable in cash;
provided, that if the applicable Change of Control was not approved by the Board of Directors of the Company, the Black Scholes Value of the remaining unexercised portion of this Warrant shall be payable at the option of the Company in either
(x) Common Stock, whereby the Company would be continually obligated to actively settle shares of Common Stock in the event insufficient authorized shares of Common Stock were available (or corresponding Corporate Event Consideration, as
applicable) valued at the value of the consideration received by the shareholders in such Change of Control or (y) cash. 
 4. NONCIRCUMVENTION.
The Company hereby covenants and agrees that the Company will not, by amendment of its certificate of incorporation or by-laws, or through any reorganization, transfer of assets, consolidation, merger, scheme,
arrangement, dissolution, issuance or sale of securities, or any other voluntary action, avoid or seek to avoid the observance or performance of any of the terms of this Warrant, and will at all times in good faith carry out all of the provisions of
this Warrant and take all commercially reasonable actions as may be required to protect the rights of the Holder. Without limiting the generality of the foregoing, the Company (i) shall not increase the par value of any shares of Common Stock
receivable upon the exercise of this Warrant above the Exercise Price then in effect, (ii) shall take all such actions as may be necessary or appropriate in order that the Company may validly and legally issue fully paid and nonassessable
shares of Common Stock upon the exercise of this Warrant, and (iii) shall, so long as any of the Warrants are outstanding, take all commercially reasonable actions necessary to reserve and keep available out of its authorized and unissued
shares of Common Stock, solely for the purpose of effecting the exercise of the Warrants, the number of shares of Common Stock as shall from time to time be necessary to effect the exercise of the Warrants then outstanding (without regard to any
limitations on exercise). 
 5. WARRANT HOLDER NOT DEEMED A STOCKHOLDER. Except as otherwise specifically provided herein, the Holder, solely in such
Person’s capacity as a holder of this Warrant, shall not be entitled to vote or receive dividends or be deemed the holder of capital stock of the Company for any purpose, nor shall anything contained in this Warrant be construed to confer upon
the Holder, solely in such Person’s capacity as the Holder of this Warrant, any of the rights of a stockholder of the Company or any right to vote, give or withhold consent to any corporate action (whether any reorganization, issue of stock,
reclassification of stock, consolidation, merger, conveyance or otherwise), receive notice of meetings, receive dividends or subscription rights, or otherwise, prior to the issuance to the Holder of the Warrant Shares which such Person is then
entitled to receive upon the due exercise of this Warrant. In addition, nothing contained in this Warrant shall be construed as imposing any liabilities on the Holder to purchase any securities (upon exercise of this Warrant or otherwise) or as a
stockholder of the Company, whether such liabilities are asserted by the Company or by creditors of the Company. 
 Notwithstanding this Section 5, the
Company shall provide the Holder with copies of the same notices and other information given to the stockholders of the Company generally, contemporaneously with the giving thereof to the stockholders; provided that the failure to deliver
such notice or any defect therein or in the delivery thereof shall not affect the validity of any corporate action required to be specified in such notice; provided, further, that the Company shall not be obligated to provide such
notice or information if it is filed with the SEC through EDGAR and available to the public through the EDGAR system. 
 6. REISSUANCE OF WARRANTS.

 (a) Transfer of Warrant. If this Warrant is to be transferred, the Holder shall surrender this Warrant to the Company, whereupon the Company will
forthwith issue and deliver upon the order of the Holder a new Warrant (in accordance with Section 6(d)), registered as the Holder may request, representing the right to purchase the number of Warrant Shares being transferred by the Holder and,
if less than the total number of Warrant Shares then underlying this Warrant is being transferred, a new Warrant (in accordance with Section 6(d)) to the Holder representing the right to purchase the number of Warrant Shares not being
transferred. 












 (b) Lost, Stolen or Mutilated Warrant. Upon receipt by the Company of evidence reasonably
satisfactory to the Company of the loss, theft, destruction or mutilation of this Warrant, and, in the case of loss, theft or destruction, of any indemnification undertaking by the Holder to the Company in customary form (but without the obligation
to post a bond) and, in the case of mutilation, upon surrender and cancellation of this Warrant, the Company shall execute and deliver to the Holder a new Warrant (in accordance with Section 6(d)) representing the right to purchase the Warrant
Shares then underlying this Warrant. 
 (c) Exchangeable for Multiple Warrants. This Warrant is exchangeable, upon the surrender hereof by the Holder
at the principal office of the Company, for a new Warrant or Warrants (in accordance with Section 6(d)) representing in the aggregate the right to purchase the number of Warrant Shares then underlying this Warrant, and each such new Warrant
will represent the right to purchase such portion of such Warrant Shares as is designated by the Holder at the time of such surrender. 
 (d) Issuance of
New Warrants. Whenever the Company is required to issue a new Warrant pursuant to the terms of this Warrant, such new Warrant (i) shall be of like tenor with this Warrant, (ii) shall represent, as indicated on the face of such new
Warrant, the right to purchase the Warrant Shares then underlying this Warrant (or in the case of a new Warrant being issued pursuant to Section 6(a) or Section 6(c), the Warrant Shares designated by the Holder which, when added to the
number of shares of Common Stock underlying the other new Warrants issued in connection with such issuance, does not exceed the number of Warrant Shares then underlying this Warrant), (iii) shall have an issuance date, as indicated on the face of
such new Warrant which is the same as the Issuance Date, and (iv) shall have the same rights and conditions as this Warrant. 
 7. NOTICES
(INCLUDING OF CERTAIN EVENTS). Whenever notice is required to be given under this Warrant, including, without limitation, an Exercise Notice, unless otherwise provided herein, such notice shall be given in writing, (i) if delivered
(a) from within the domestic United States, by first-class registered or certified airmail, or nationally recognized overnight express courier, postage prepaid, electronic mail or by facsimile or (b) from outside the United States, by
International Federal Express, electronic mail or facsimile, and (ii) will be deemed given (A) if delivered by first-class registered or certified mail domestic, three (3) Business Days after so mailed, (B) if delivered by
nationally recognized overnight carrier, one (1) Business Day after so mailed, (C) if delivered by International Federal Express, two (2) Business Days after so mailed and (D) at the time of transmission, if delivered by
electronic mail to the email address specified in this Section 7 prior to 5:00 p.m. (New York time) on a Trading Day, (E) the next Trading Day after the date of transmission, if delivered by electronic mail to the email address specified
in this Section 7 on a day that is not a Trading Day or later than 5:00 p.m. (New York time) on any Trading Day and (F) if delivered by facsimile, upon electronic confirmation of delivery of such facsimile, and will be delivered and
addressed as follows: 
 (i) If to the Company, to: 

X4 Pharmaceuticals, Inc. 
 955
Massachusetts Avenue, 4th Floor 
 Cambridge, MA 02139 

Attention: Chief Financial Officer 

Email: adam.mostafa@x4pharma.com 

with a copy (which shall not constitute notice) to: 

Cooley LLP 
 55 Hudson Yards 

New York, NY 10001 
 Attention:
Daniel I. Goldberg, Esq. 
 Facsimile: (617) 542-2241 

Email: dgoldberg@cooley.com 

(ii) if to the Holder, at such address or other contact information delivered by the Holder to Company or as is on the books
and records of the Company. 
 The Company shall provide the Holder with prompt written notice of all actions taken pursuant to this Warrant, including in
reasonable detail a description of such action and the reason therefor. Without limiting the generality of the foregoing, the Company will give written notice to the Holder (i) immediately upon any adjustment of the Exercise Price, setting
forth in reasonable detail, and certifying, the calculation of such adjustment; (ii) at least ten (10) days prior to the date on which the Company closes its books or takes a record (A) with respect to any dividend or distribution
upon the shares of Common Stock, (B) with respect to any grants, issuances or sales of any Options, Convertible Securities or rights to purchase stock, warrants, securities or other property, in each case pro rata to the record holders of
Common Stock or (C) for determining rights to vote with respect to any Fundamental Transaction, dissolution or liquidation and (iii) within two (2) Business Days 












 
following the Data Release Date; provided in each case that such information shall be made known to the public prior to or in conjunction with such notice being provided to the Holder;
provided, further, that the failure to deliver such notice or any defect therein or in the delivery thereof shall not affect the validity of the corporate action required to be specified in such notice. 

8. AMENDMENT AND WAIVER. Except as otherwise provided herein, the provisions of this Warrant may be amended or waived and the Company may take any
action herein prohibited, or omit to perform any act herein required to be performed by it, only if the Company has obtained the written consent of the Holder. 

9. GOVERNING LAW; JURY TRIAL. This Warrant shall be governed by and construed and enforced in accordance with, and all questions concerning the
construction, validity, interpretation and performance of this Warrant shall be governed by, the internal laws of the State of New York, without giving effect to any choice of law or conflict of law provision or rule (whether of the State of New
York or any other jurisdictions) that would cause the application of the laws of any jurisdictions other than the State of New York. If either party shall commence an action, suit or proceeding to enforce any provisions of this Warrant, the
prevailing party in such action, suit or proceeding shall be reimbursed by the other party for their reasonable attorneys’ fees and other costs and expenses incurred with the investigation, preparation and prosecution of such action or
proceeding. THE COMPANY HEREBY IRREVOCABLY WAIVES ANY RIGHT IT MAY HAVE, AND AGREES NOT TO REQUEST, A JURY TRIAL FOR THE ADJUDICATION OF ANY DISPUTE HEREUNDER OR IN CONNECTION WITH OR ARISING OUT OF THIS WARRANT OR ANY TRANSACTION CONTEMPLATED
HEREBY. 
 10. DISPUTE RESOLUTION. In the case of a dispute as to the determination of the Exercise Price or the arithmetic calculation of the
Warrant Shares, the Company shall submit the disputed determinations or arithmetic calculations via facsimile or electronic mail within two (2) Business Days of receipt of the Exercise Notice or other event giving rise to such dispute, as the
case may be, to the Holder. If the Holder and the Company are unable to agree upon such determination or calculation of the Exercise Price or the Warrant Shares within three (3) Business Days of such disputed determination or arithmetic
calculation being submitted to the Holder, then the Company shall, within two (2) Business Days submit via facsimile or electronic mail (a) the disputed determination of the Exercise Price to an independent, reputable investment bank
selected by the Company and approved by the Holder or (b) the disputed arithmetic calculation of the Warrant Shares to the Company’s independent, outside accountant. The Company shall cause at its expense the investment bank or the
accountant, as the case may be, to perform the determinations or calculations and notify the Company and the Holder of the results no later than ten (10) Business Days from the time it receives the disputed determinations or calculations. Such
investment bank’s or accountant’s determination or calculation, as the case may be, shall be binding upon all parties absent demonstrable error. 

11. REMEDIES, OTHER OBLIGATIONS, BREACHES AND INJUNCTIVE RELIEF. The remedies provided in this Warrant shall be cumulative and in addition to all other
remedies available under this Warrant, at law or in equity (including a decree of specific performance and/or other injunctive relief), and nothing herein shall limit the right of the Holder to pursue actual damages for any failure by the Company to
comply with the terms of this Warrant. The Company acknowledges that a breach by it of its obligations hereunder will cause irreparable harm to the Holder and that the remedy at law for any such breach may be inadequate. The Company therefore agrees
that, in the event of any such breach or threatened breach, the holder of this Warrant shall be entitled, in addition to all other available remedies, to seek an injunction restraining any breach, without the necessity of showing economic loss and
without any bond or other security being required. 
 12. TRANSFER. Subject to compliance with applicable federal and state securities laws, this
Warrant and the Warrant Shares may be offered for sale, sold, transferred, pledged or assigned without the consent of the Company. 
 13. SEVERABILITY;
CONSTRUCTION; HEADINGS. If any provision of this Warrant is prohibited by law or otherwise determined to be invalid or unenforceable by a court of competent jurisdiction, the provision that would otherwise be prohibited, invalid or unenforceable
shall be deemed amended to apply to the broadest extent that it would be valid and enforceable, and the invalidity or unenforceability of such provision shall not affect the validity of the remaining provisions of this Warrant so long as this
Warrant as so modified continues to express, without material change, the original intentions of the parties as to the subject matter hereof and the prohibited nature, invalidity or unenforceability of the provision(s) in question does not
substantially impair the respective expectations or reciprocal obligations of the parties or the practical realization of the benefits that would otherwise be conferred upon the parties. The parties will endeavor in good faith negotiations to
replace the prohibited, invalid or unenforceable provision(s) with a valid provision(s), the effect of which comes as close as possible to that of the prohibited, invalid or unenforceable provision(s). This Warrant shall be deemed to be jointly
drafted by the Company and the Holder and shall not be construed against any Person as the drafter hereof. The headings of this Warrant are for convenience of reference and shall not form part of, or affect the interpretation of, this Warrant. 












 14. DISCLOSURE. In the event that the Company believes that a notice delivered by the Company in
accordance with the terms of this Warrant contains material, nonpublic information relating to the Company or its subsidiaries, the Company so shall indicate to such Holder contemporaneously with delivery of such notice, and in the absence of any
such indication, the Holder shall be allowed to presume that all matters relating to such notice do not constitute material, nonpublic information relating to the Company or its subsidiaries. 

15. CERTAIN DEFINITIONS. For purposes of this Warrant, the following terms shall have the following meanings: 

(a) “Affiliate” means any Person that, directly or indirectly through one or more intermediaries, controls or is controlled by or is under
common control with a Person, as such terms are used in and construed under Rule 405 under the Securities Act of 1933, as amended. 
 (b)
“Attribution Parties” means, collectively, the following Persons and entities: (i) any investment vehicle, including, any funds, feeder funds or managed accounts, currently, or from time to time after the Subscription Date,
directly or indirectly managed or advised by the Holder’s investment manager or any of its Affiliates or principals, (ii) any direct or indirect Affiliates of the Holder or any of the foregoing, (iii) any Person acting or who could be
deemed to be acting as a Group together with the Holder or any of the foregoing and (iv) any other Persons whose beneficial ownership of the Company’s Common Stock would or could be aggregated with the Holder’s and the other
Attribution Parties for purposes of Section 13(d) or Section 16 of the 1934 Act. For clarity, the purpose of the foregoing is to subject collectively the Holder and all other Attribution Parties to the Maximum Percentage. 

(c) “Bid Price” means, for any security as of the particular time of determination, the bid price for such security on the Principal Market
as reported by Bloomberg as of such time of determination, or, if the Principal Market is not the principal securities exchange or trading market for such security, the bid price of such security on the principal securities exchange or trading
market where such security is listed or traded as reported by Bloomberg as of such time of determination, or if the foregoing does not apply, the bid price of such security in the
over-the-counter market on the electronic bulletin board for such security as reported by Bloomberg as of such time of determination, or, if no bid price is reported for
such security by Bloomberg as of such time of determination, the average of the bid prices of any market makers for such security as reported in the “pink sheets” by OTC Markets Group Inc. (formerly Pink Sheets LLC) as of such time of
determination. If the Bid Price cannot be calculated for a security as of the particular time of determination on any of the foregoing bases, the Bid Price of such security as of such time of determination shall be the fair market value as mutually
determined by the Company and the Holder. If the Company and the Holder are unable to agree upon the fair market value of such security, then such dispute shall be resolved in accordance with the procedures in Section 10. All such
determinations shall be appropriately adjusted for any stock dividend, stock split, stock combination or other similar transaction during such period. 

(d) “Black Scholes Value” means the value of this Warrant based on the Black-Scholes Option Pricing Model obtained from the “OV”
function on Bloomberg determined as of the day immediately following the first public announcement of the applicable Change of Control, or, if the Change of Control is not publicly announced, the date the Change of Control is consummated, for
pricing purposes and reflecting (i) a risk-free interest rate corresponding to the U.S. Treasury rate for a period equal to the remaining term of this Warrant as of such date of request, (ii) an expected volatility equal to 50%, (iii) the
underlying price per share used in such calculation shall be the greater of (A) the sum of the price per share being offered in cash, if any, plus the per share value of any non-cash consideration, if
any, being offered in such Change of Control and (B) the greater of (x) the last Weighted Average Price immediately prior to the public announcement of such Change of Control and (y) the last Weighted Average Price immediately prior
to the consummation of such Change of Control, (iv) a zero cost of borrow and (v) a 360 day annualization factor. 
 (e)
“Bloomberg” means Bloomberg Financial Markets. 
 (f) “Business Day” means any day other than Saturday, Sunday or other
day on which commercial banks in The City of New York are authorized or required by law to remain closed. 
 (g) “Change of Control” means
any Fundamental Transaction other than (i) any reorganization, recapitalization or reclassification of the shares of Common Stock in which holders of the Company’s voting power immediately prior to such reorganization, recapitalization or
reclassification continue after such reorganization, recapitalization or reclassification to hold publicly traded securities and, directly or indirectly, are, in all material respects, the holders of the voting power of the surviving entity (or
entities with the authority or voting power to elect the members of the board of directors (or their equivalent if other than a corporation) of such entity or entities) after such reorganization, recapitalization or reclassification,
(ii) pursuant to a migratory merger effected solely for the purpose of changing the jurisdiction of incorporation of the Company or (iii) a merger in connection with a bona fide acquisition by the Company of any Person in which
(x) the gross 












 
consideration paid, directly or indirectly, by the Company in such acquisition is not greater than 20% of the Company’s market capitalization as calculated on the date of the consummation of
such merger and (y) such merger does not contemplate a change to the identity of a majority of the board of directors of the Company. Notwithstanding anything herein to the contrary, any transaction or series of transactions that, directly or
indirectly, results in the Company or the Successor Entity not having Common Stock or common stock, as applicable, registered under the 1934 Act and listed on an Eligible Market shall be deemed a Change of Control. 

(h) “Closing Bid Price” and “Closing Sale Price” means, for any security as of any date, the last closing bid price and last
closing trade price, respectively, for such security on the Principal Market, as reported by Bloomberg, or, if the Principal Market begins to operate on an extended hours basis and does not designate the closing bid price or the closing trade price,
as the case may be, then the last bid price or the last trade price, respectively, of such security prior to 4:00:00 p.m., New York time, as reported by Bloomberg, or, if the Principal Market is not the principal securities exchange or trading
market for such security, the last closing bid price or last trade price, respectively, of such security on the principal securities exchange or trading market where such security is listed or traded as reported by Bloomberg, or if the foregoing do
not apply, the last closing bid price or last trade price, respectively, of such security in the over-the-counter market on the electronic bulletin board for such
security as reported by Bloomberg, or, if no closing bid price or last trade price, respectively, is reported for such security by Bloomberg, the average of the bid prices, or the ask prices, respectively, of any market makers for such security as
reported in the OTC Link or “pink sheets” by OTC Markets Group Inc. (formerly Pink OTC Markets Inc.). If the Closing Bid Price or the Closing Sale Price cannot be calculated for a security on a particular date on any of the foregoing
bases, the Closing Bid Price or the Closing Sale Price, as the case may be, of such security on such date shall be the fair market value as mutually determined by the Company and the Holder. If the Company and the Holder are unable to agree upon the
fair market value of such security, then such dispute shall be resolved pursuant to Section 10. All such determinations to be appropriately adjusted for any stock dividend, stock split, stock combination, reclassification or other similar
transaction during the applicable calculation period. 
 (i) “Common Stock” means (i) the Company’s Common Stock, par value
$0.001 per share, and (ii) any capital stock into which such Common Stock shall have been changed or any capital stock resulting from a reclassification of such Common Stock. 

(j) “Convertible Securities” means any stock or securities (other than Options) directly or indirectly convertible into or exercisable or
exchangeable for shares of Common Stock. 
 (k) “Eligible Market” means The Nasdaq Capital Market, the NYSE American LLC, The Nasdaq Global
Select Market, The Nasdaq Global Market or The New York Stock Exchange, Inc. 
 (l) “Expiration Date” means the earlier of (a) the
date that is thirty (30) calendar days following the date on which the Company issues a press release regarding topline data from its Phase III clinical trial of mavorixafor for the treatment of WHIM syndrome (the “Data Release
Date”) (or, if such date is not a Business Day, then the next Business Day following such date) and (b) the date that is sixty (60) months from the Issuance Date. 

(m) “Fundamental Transaction” means (A) that the Company shall, directly or indirectly, including through subsidiaries, Affiliates or
otherwise, in one or more related transactions, (i) consolidate or merge with or into (whether or not the Company is the surviving corporation) another Subject Entity, or (ii) sell, assign, transfer, convey or otherwise dispose of all or
substantially all of the properties or assets of the Company or any of its “significant subsidiaries” (as defined in Rule 1-02 of Regulation S-X), taken as a
whole, to one or more Subject Entities, or (iii) make, or allow one or more Subject Entities to make, or allow the Company to be subject to or have its shares of Common Stock be subject to or party to one or more Subject Entities making, a
purchase, tender or exchange offer that is accepted by the holders of at least either (x) 50% of the outstanding shares of Common Stock, (y) 50% of the outstanding shares of Common Stock calculated as if any shares of Common Stock held by all
Subject Entities making or party to, or Affiliated with any Subject Entities making or party to, such purchase, tender or exchange offer were not outstanding; or (z) such number of shares of Common Stock such that all Subject Entities making or
party to, or Affiliated with any Subject Entity making or party to, such purchase, tender or exchange offer, become collectively the beneficial owners (as defined in Rule 13d-3 under the 1934 Act) of at least
50% of the outstanding shares of Common Stock, or (iv) consummate a stock purchase agreement or other business combination (including, without limitation, a reorganization, recapitalization, spin-off or
scheme of arrangement) with one or more Subject Entities whereby all such Subject Entities, individually or in the aggregate, acquire, either (x) at least 50% of the outstanding shares of Common Stock, (y) at least 50% of the outstanding
shares of Common Stock calculated as if any shares of Common Stock held by all the Subject Entities making or party to, or Affiliated with any Subject Entity making or party to, such stock purchase agreement or other business combination were not
outstanding; or (z) such number of shares of Common Stock such that the Subject Entities become collectively the beneficial owners (as defined in Rule 13d-3 under the 1934 Act) of at least 50% of the
outstanding shares of Common Stock, or (v) reorganize, recapitalize or reclassify its shares of Common Stock, (B) that the Company shall, directly or indirectly, including through subsidiaries, Affiliates or otherwise,













 
in one or more related transactions, allow any Subject Entity individually or the Subject Entities in the aggregate to be or become the “beneficial owner” (as defined in Rule 13d-3 under the 1934 Act), directly or indirectly, whether through acquisition, purchase, assignment, conveyance, tender, tender offer, exchange, reduction in outstanding shares of Common Stock, merger,
consolidation, business combination, reorganization, recapitalization, spin-off, scheme of arrangement, reorganization, recapitalization or reclassification or otherwise in any manner whatsoever, of either
(x) at least 50% of the aggregate ordinary voting power represented by issued and outstanding shares of Common Stock, (y) at least 50% of the aggregate ordinary voting power represented by issued and outstanding shares of Common Stock not
held by all such Subject Entities as of the Subscription Date calculated as if any shares of Common Stock held by all such Subject Entities were not outstanding, or (z) a percentage of the aggregate ordinary voting power represented by issued
and outstanding shares of Common Stock or other equity securities of the Company sufficient to allow such Subject Entities to effect a statutory short form merger or other transaction requiring other stockholders of the Company to surrender their
Common Stock without approval of the stockholders of the Company or (C) directly or indirectly, including through subsidiaries, Affiliates or otherwise, in one or more related transactions, the issuance of or the entering into any other
instrument or transaction structured in a manner to circumvent, or that circumvents, the intent of this definition in which case this definition shall be construed and implemented in a manner otherwise than in strict conformity with the terms of
this definition to the extent necessary to correct this definition or any portion of this definition which may be defective or inconsistent with the intended treatment of such instrument or transaction. 

(n) “Group” means a “group” as that term is used in Section 13(d) of the 1934 Act and as defined in Rule 13d-5 thereunder. 
 (o) “Options” means any rights, warrants or options to subscribe for or purchase
shares of Common Stock or Convertible Securities. 
 (p) “Parent Entity” of a Person means an entity that, directly or indirectly, controls
the applicable Person, including such entity whose common stock or equivalent equity security is quoted or listed on an Eligible Market, or, if there is more than one such Person or such entity, the Person or such entity designated by the Holder or
in the absence of such designation, such Person or entity with the largest public market capitalization as of the date of consummation of the Fundamental Transaction. 

(q) “Person” means an individual, a limited liability company, a partnership, a joint venture, a corporation, a trust, an unincorporated
organization, any other entity and a government or any department or agency thereof. 
 (r) “Principal Market” means the Nasdaq Capital
Market. 
 (s) “Standard Settlement Period” means the standard settlement period, expressed in a number of Trading Days, for the
Company’s primary trading market or quotation system with respect to the Common Stock that is in effect on the date of delivery of an applicable Exercise Notice, which as of the Issuance Date was “T+2”. 

(t) “Subject Entity” means any Person, Persons or Group or any Affiliate or associate of any such Person, Persons or Group. 

(u) “Successor Entity” means one or more Person or Persons formed by, resulting from or surviving any Fundamental Transaction or one or more
Person or Persons with which such Fundamental Transaction shall have been entered into, or in each, case the resulting Parent Entity. 
 (v)
“Trading Day” means any day on which the Common Stock is traded on the Principal Market, or, if the Principal Market is not the principal trading market for the Common Stock, then on the principal securities exchange or securities
market on which the Common Stock is then traded. 
 (w) “Weighted Average Price” means, for any security as of any date, the dollar
volume-weighted average price for such security on the Principal Market during the period beginning at 9:30:01 a.m., New York time (or such other time as the Principal Market publicly announces is the official open of trading), and ending at 4:00:00
p.m., New York time (or such other time as the Principal Market publicly announces is the official close of trading), as reported by Bloomberg through its “Volume at Price” function or, if the foregoing does not apply, the dollar
volume-weighted average price of such security in the over-the-counter market on the electronic bulletin board for such security during the period beginning at 9:30:01
a.m., New York time (or such other time as such market publicly announces is the official open of trading), and ending at 4:00:00 p.m., New York time (or such other time as such market publicly announces is the official close of trading), as
reported by Bloomberg, or, if no dollar volume-weighted average price is reported for such security by Bloomberg for such hours, the average of the highest Closing Bid Price and the lowest closing ask price of any of the market makers for such
security as reported in the OTC Link or “pink sheets” by OTC Markets Group Inc. (formerly Pink OTC Markets Inc.). If the Weighted Average Price cannot be calculated for a security on a particular date on any of the foregoing bases, the
Weighted Average Price of such security on such date shall be the fair market value as mutually determined by the Company and the Holder. If the Company and the Holder are unable to agree upon the fair market value of such security, then such
dispute shall be resolved pursuant to Section 10 but with the term “Weighted Average Price” being substituted for the term “Exercise Price.” All such determinations shall be appropriately adjusted for any stock dividend,
stock split, stock combination, reclassification or other similar transaction during the applicable calculation period. 
 [Signature Page
Follows] 












 IN WITNESS WHEREOF, the Company has caused this Class B Warrant to Purchase
Common Stock to be duly executed as of the Issuance Date set out above. 
  

			
	 X4 PHARMACEUTICALS, INC.

		
	 By:
	 	
	 Name:
	 	
	 Title:
	 	














 EXHIBIT A 

EXERCISE NOTICE 
 TO BE
EXECUTED BY THE REGISTERED HOLDER TO EXERCISE THIS CLASS B 
 WARRANT TO PURCHASE COMMON STOCK 

X4 PHARMACEUTICALS, INC. 
 The undersigned
holder hereby exercises the right to purchase shares of Common Stock (“Warrant Shares”) of X4 Pharmaceuticals, Inc., a Delaware corporation (the “Company”), evidenced by the attached Class B Warrant to Purchase
Common Stock (the “Warrant”). Capitalized terms used herein and not otherwise defined shall have the respective meanings set forth in the Warrant. 

1. Form of Exercise. The Holder intends that payment of the Exercise Price shall be made as: 

a “Cash Exercise” with respect to Warrant Shares; 
 a
“Cashless Exercise” with respect to Warrant Shares; 
 a “Cash Exercise” with respect to
Pre-Funded Warrants; and/or 
 a “Cashless Exercise” with respect to
Pre-Funded Warrants 
 2. Payment of Exercise Price. In the event that the holder has elected a Cash Exercise
with respect to some or all of the Warrant Shares and/or Pre-Funded Warrants to be issued pursuant hereto, the holder shall pay the Aggregate Exercise Price in the sum of
$                to the Company in accordance with the terms of the Warrant. 

3. Delivery of Warrant Shares and/or Pre-Funded Warrants. The Company shall deliver to the holder Warrant
Shares and/or Pre-Funded Warrants in accordance with the terms of the Warrant. 
 Date: 

 

			
	 Name of Registered Holder

		
	 By:
	 	
		 	 Name:

		 	 Title:














 ACKNOWLEDGMENT 

The Company hereby acknowledges this Exercise Notice and hereby directs Computershare Trust Company, N.A. to issue the above indicated number of shares of
Common Stock on or prior to the applicable Share Delivery Date. 
  

			
	 X4 PHARMACEUTICALS, INC.

		
	 By:
	 	
	 Name:
	 	
	 Title: