<PAGE>   1
                                                                    Exhibit 4.11


================================================================================


                      COMMON SECURITIES GUARANTEE AGREEMENT

                             BOSTONFED BANCORP, INC.

                         Dated as of September 22, 2000


================================================================================

<PAGE>   2

                                TABLE OF CONTENTS
                                                                            PAGE
                                    ARTICLE I

                         DEFINITIONS AND INTERPRETATION

SECTION 1.1.  DEFINITIONS AND INTERPRETATION...................................3

                                   ARTICLE II

                                    GUARANTEE

SECTION 2.1.  GUARANTEE........................................................4
SECTION 2.2.  WAIVER OF NOTICE AND DEMAND......................................4
SECTION 2.3.  OBLIGATIONS NOT AFFECTED.........................................4
SECTION 2.4.  RIGHTS OF HOLDERS................................................5
SECTION 2.5.  GUARANTEE OF PAYMENT.............................................5
SECTION 2.6.  SUBROGATION......................................................5
SECTION 2.7.  INDEPENDENT OBLIGATIONS..........................................6

                                   ARTICLE III

                    LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 3.1.  LIMITATION OF TRANSACTIONS.......................................6
SECTION 3.2.  RANKING..........................................................7

                                   ARTICLE IV

                                   TERMINATION
SECTION 4.1. TERMINATION.......................................................7

                                    ARTICLE V

                                  MISCELLANEOUS

SECTION 5.1.  SUCCESSORS AND ASSIGNS...........................................7
SECTION 5.2.  AMENDMENTS.......................................................7
SECTION 5.3.  NOTICES..........................................................8
SECTION 5.4.  BENEFIT..........................................................9
SECTION 5.5.  GOVERNING LAW....................................................9

<PAGE>   3

                      COMMON SECURITIES GUARANTEE AGREEMENT

     This COMMON SECURITIES GUARANTEE AGREEMENT (the "Common Securities
Guarantee"), dated as of September 22, 2000, is executed and delivered by
BostonFed Bancorp, Inc., a Delaware corporation (the "Guarantor"), for the
benefit of the Holders (as defined herein) from time to time of the Common
Securities (as defined herein) of BFD Preferred Capital Trust II, a Delaware
business trust (the "Issuer").

     WHEREAS, pursuant to an Amended and Restated Declaration of Trust (the
"Declaration"), dated as of September 22, 2000, by and among the Trustees of the
Issuer named therein, the Guarantor, as sponsor, and the holders from time to
time of undivided beneficial interests in the assets of the Issuer, the Issuer
is issuing on the date hereof 681 common securities having an aggregate
liquidation amount of Six Hundred Eighty-One Thousand Dollars ($681,000), to
meet the capital requirements of the Trust; such common securities being
designated the 10.875% Common Securities (the "Common Securities").

     WHEREAS, as incentive for the Holders to purchase the Common Securities,
the Guarantor desires irrevocably and unconditionally to agree, to the extent
set forth in this Common Securities Guarantee, to pay the Guarantee Payments (as
defined herein) to the Holders of the Common Securities and to make certain
other payments on the terms and conditions set forth herein; and

     WHEREAS, the Guarantor is also executing and delivering the Series A
Capital Securities Guarantee Agreement, dated as of September 22, 2000 (the
"Series A Capital Securities Guarantee"), for the benefit of the holders of the
Series A Capital Securities (as defined in the Declaration) and upon
consummation of the Exchange Offer (as defined in the Declaration) will execute
and deliver the Series B Capital Securities Guarantee Agreement (the "Series B
Capital Securities Guarantee") for the benefit of the holders of the Series B
Capital Securities (as defined in the Declaration); it being understood that if
an Event of Default (as defined in the Declaration) has occurred and is
continuing, the rights of Holders of the Common Securities to receive Guarantee
Payments under this Common Securities Guarantee are subordinated, to the extent
and in the manner set forth herein, to the rights of holders of Capital
Securities (as defined in the Declaration) to receive Guarantee Payments under
the Series A Capital Securities Guarantee and the Series B Capital Securities
Guarantee, as the case may be.

     NOW, THEREFORE, in consideration of the purchase by each Holder of the
Common Securities, which purchase the Guarantor hereby acknowledges shall
benefit the Guarantor, the Guarantor executes and delivers this Common
Securities Guarantee for the benefit of the Holders.


                                       2
<PAGE>   4

                                    ARTICLE I
                         DEFINITIONS AND INTERPRETATION

SECTION 1.1. DEFINITIONS AND INTERPRETATION

     In this Common Securities Guarantee, unless the context otherwise requires:

     (a)  capitalized terms used in this Common Securities Guarantee but not
defined in the preamble above have the respective meanings assigned to them in
this Section 1.1;

     (b)  terms defined in the Declaration as at the date of execution of this
Common Securities Guarantee have the same meaning when used in this Common
Securities Guarantee unless otherwise defined in this Common Securities
Guarantee;

     (c)  a term defined anywhere in this Common Securities Guarantee has the
same meaning throughout;

     (d)  all references to "the Common Securities Guarantee" or "this Common
Securities Guarantee" are references to this Common Securities Guarantee as
modified, supplemented or amended from time to time;

     (e)  all references in this Common Securities Guarantee to Articles and
Sections are references to Articles and Sections of this Common Securities
Guarantee unless otherwise specified;

     (f)  a term defined in the Trust Indenture Act has the same meaning as in
the Trust Indenture Act unless otherwise defined in this Common Securities
Guarantee; and

     (g)  a reference to the singular includes the plural and vice versa.

     "Guarantee Payments" means the following payments or distributions, without
duplication, with respect to the Common Securities, to the extent not paid or
made by or on behalf of the Issuer: (i) any accumulated and unpaid Distributions
(as defined in the Declaration) that are required to be paid on such Common
Securities, to the extent the Issuer has funds legally available therefor at
such time, (ii) the redemption price, including all accumulated and unpaid
Distributions to the date of redemption (the "Redemption Price"), to the extent
the Issuer has funds legally available therefor at such time, with respect to
any Common Securities called for redemption, and (iii) upon a voluntary or
involuntary dissolution, winding-up or liquidation of the Issuer (other than in
connection with the distribution of Debentures (as defined in the Declaration)
to the Holders in exchange for Common Securities or in connection with the
redemption of the Common Securities, in each case as provided in the
Declaration), the lesser of (a) the aggregate of the liquidation amount and all
accumulated and unpaid Distributions on the Common Securities to the date of
payment, to the extent the Issuer has funds legally available therefor at such
time, and (b) the amount of assets of the Issuer remaining available for
distribution to the Holders after satisfaction of liabilities to creditors of
the Issuer as required by applicable law (in either case, the "Liquidation
Distribution"). If an Event of Default under the Declaration, the Series A
Capital Securities Guarantee Agreement or the Series B Capital Securities
Guarantee Agreement has occurred and is continuing, no Guarantee

                                       3
<PAGE>   5

Payments with respect to the Common Securities shall be made until holders of
Capital Securities shall be paid in full the Guarantee Payments to which they
are entitled under the Series A Capital Securities Guarantee and the Series B
Capital Securities Guarantee, as the case may be.

     "Holder" means any holder, as registered on the books and records of the
Issuer, of any Common Securities.

     "Other Guarantees" means all guarantees, other than this Common Securities
Guarantee, to be issued by the Guarantor with respect to common securities (if
any) similar to the Common Securities issued by trusts other than the Issuer to
be established by the Guarantor (if any), in each case similar to the Issuer.


                                   ARTICLE II
                                    GUARANTEE

SECTION 2.1. GUARANTEE

     The Guarantor irrevocably and unconditionally agrees to pay in full to the
Holders the Guarantee Payments (without duplication of amounts theretofore paid
by the Issuer), as and when due, regardless of any defense, right of set-off or
counterclaim which the Issuer may have or assert. The Guarantor's obligation to
make a Guarantee Payment may be satisfied by direct payment of the required
amounts by the Guarantor to the Holders or by causing the Issuer to pay such
amounts to the Holders.

SECTION 2.2. WAIVER OF NOTICE AND DEMAND

     The Guarantor hereby waives notice of acceptance of this Common Securities
Guarantee and of any liability to which it applies or may apply, presentment,
demand for payment, any right to require a proceeding first against the Issuer
or any other Person before proceeding against the Guarantor, protest, notice of
nonpayment, notice of dishonor, notice of redemption and all other notices and
demands.

SECTION 2.3. OBLIGATIONS NOT AFFECTED

     The obligations, covenants, agreements and duties of the Guarantor under
this Common Securities Guarantee shall in no way be affected or impaired by
reason of the happening from time to time of any of the following:

     (a)  the release or waiver, by operation of law or otherwise, of the
performance or observance by the Issuer of any express or implied agreement,
covenant, term or condition relating to the Common Securities to be performed or
observed by the Issuer;

     (b)  the extension of time for the payment by the Issuer of all or any
portion of the Distributions, Redemption Price, Liquidation Distribution or any
other sums payable under the terms of the Common Securities or the extension of
time for the performance of any other obligation under, arising out of, or in
connection with, the Common Securities;

                                       4
<PAGE>   6

     (c)  any failure, omission, delay or lack of diligence on the part of the
Holders to enforce, assert or exercise any right, privilege, power or remedy
conferred on the Holders pursuant to the terms of the Common Securities, or any
action on the part of the Issuer granting indulgence or extension of any kind;

     (d)  the voluntary or involuntary liquidation, dissolution, sale of any
collateral, receivership, insolvency, bankruptcy, assignment for the benefit of
creditors, reorganization, arrangement, composition or readjustment of debt of,
or other similar proceedings affecting, the Issuer or any of the assets of the
Issuer;

     (e)  any invalidity of, or defect or deficiency in, the Common Securities;

     (f)  the settlement or compromise of any obligation guaranteed hereby or
hereby incurred; or

     (g)  any other circumstance whatsoever that might otherwise constitute a
legal or equitable discharge or defense of a guarantor;

it being the intent of this Section 2.3 that the obligations of the Guarantor
with respect to the Guarantee Payments shall be absolute and unconditional under
any and all circumstances.

     There shall be no obligation of the Holders to give notice to, or obtain
consent of, the Guarantor with respect to the happening of any of the foregoing.

SECTION 2.4. RIGHTS OF HOLDERS

     The Guarantor expressly acknowledges that any Holder of the Common
Securities may institute a legal proceeding directly against the Guarantor to
enforce its rights under this Common Securities Guarantee, without first
instituting a legal proceeding against the Issuer or any other Person.

SECTION 2.5. GUARANTEE OF PAYMENT

     This Common Securities Guarantee creates a guarantee of payment and not of
collection.

SECTION 2.6. SUBROGATION

     The Guarantor shall be subrogated to all (if any) rights of the Holders of
the Common Securities against the Issuer in respect of any amounts paid to such
Holders by the Guarantor under this Common Securities Guarantee; PROVIDED,
HOWEVER, that the Guarantor shall not (except to the extent required by
mandatory provisions of law) be entitled to enforce or exercise any rights which
it may acquire by way of subrogation or any indemnity, reimbursement or other
agreement, in all cases as a result of payment under this Common Securities
Guarantee, if, at the time of any such payment, any amounts are due and unpaid
under this Common Securities Guarantee. If any amount

                                       5
<PAGE>   7

shall be paid to the Guarantor in violation of the preceding sentence, the
Guarantor agrees to hold such amount in trust for the Holders and to pay over
such amount to the Holders.

SECTION 2.7. INDEPENDENT OBLIGATIONS

     The Guarantor acknowledges that its obligations hereunder are independent
of the obligations of the Issuer with respect to the Common Securities and that
the Guarantor shall be liable as principal and as debtor hereunder to make
Guarantee Payments pursuant to the terms of this Common Securities Guarantee
notwithstanding the occurrence of any event referred to in subsections (a)
through (g), inclusive, of Section 2.3 hereof.


                                   ARTICLE III
                    LIMITATION OF TRANSACTIONS; SUBORDINATION

SECTION 3.1.  LIMITATION OF TRANSACTIONS

     So long as any Common Securities remain outstanding, the Guarantor will not
(i) declare or pay any dividends or distribution on, or redeem, purchase,
acquire or make a liquidation payment with respect to any of the Guarantor's
capital stock, (ii) make any payment of principal of, or interest or premium, if
any, on or repay, repurchase or redeem any debt securities of the Guarantor
(including Other Debentures, as defined in the Indenture) that rank pari passu
with or junior in right of payment to the Debentures or (iii) make any guarantee
payments with respect to any guarantee by the Guarantor of the debt securities
of any subsidiary of the Guarantor (including Other Guarantees, as defined in
the Indenture) if such guarantee ranks pari passu with or junior in right of
payment to the Debentures (other than (a) dividends or distributions in shares
of, or options, warrants or rights to subscribe for or purchase shares of,
common stock of the Guarantor, (b) any declaration of a dividend in connection
with the implementation of a shareholders' rights plan, or the issuance of stock
under any such plan in the future, or the redemption or repurchase of any such
rights pursuant thereto, (c) payments under the Series A Capital Securities
Guarantee and the Series B Capital Securities Guarantee, (d) as a result of a
reclassification of the Guarantor's capital stock or the exchange or the
conversion of one class or series of the Guarantor's capital stock for another
class or series of the Guarantor's capital stock, (e) the purchase of fractional
interests in shares of the Guarantor's capital stock pursuant to the conversion
or exchange provisions of such capital stock or the security being converted or
exchanged, and (f) purchases of common stock related to the issuance of common
stock or rights under any of the Guarantor's benefit or compensation plans for
its directors, officers or employees or any of the Guarantor's dividend
reinvestment plans) if at such time (1) there shall have occurred any event of
which the Guarantor has actual knowledge that (A) is, or with the giving of
notice or the lapse of time, or both, would be, an Event of Default and (B) in
respect of which the Guarantor shall not have taken reasonable steps to cure,
(2) if such Debentures are held by the Property Trustee, the Guarantor shall be
in default with respect to its payment of any obligations under the Series A
Capital Securities Guarantee or the Series B Capital Securities Guarantee or (3)
the Guarantor shall have given notice of its election of the exercise of its
right to commence an Extended Interest Payment Period as provided in the
Indenture and shall not have rescinded such notice, and such Extended Interest
Payment Period, or an extension thereof, shall have commenced and be continuing.

                                       6
<PAGE>   8

SECTION 3.2. RANKING

     This Common Securities Guarantee will constitute an unsecured obligation of
the Guarantor and will rank (i) subordinate and junior in right of payment to
the Senior Indebtedness (as defined in the Indenture), to the same extent and in
the same manner that the Debentures are subordinated to the Senior Indebtedness
pursuant to the Indenture, it being understood that the terms of Article XV of
the Indenture shall apply to the obligations of the Guarantor under this Common
Securities Guarantee as if such Article XV were set forth herein in full, (ii)
pari passu with the most senior preferred or preference stock now or hereafter
issued by the Guarantor and with any Other Guarantee and, except as set forth
herein or therein, the Series A Capital Securities Guarantee, the Series B
Capital Securities Guarantee and any guarantee now or hereafter entered into by
the Guarantor in respect of any preferred or preference stock of any Affiliate
of the Guarantor and (iii) senior to the Guarantor's common stock.


                                   ARTICLE IV
                                   TERMINATION

SECTION 4.1. TERMINATION

     This Common Securities Guarantee shall terminate and be of no further force
or effect upon (i) full payment of the Redemption Price of all Common
Securities, (ii) the dissolution, winding-up or liquidation of the Issuer,
immediately following the full payment of the amounts payable in accordance with
the Declaration, or (iii) the distribution of all of the Debentures to the
Holders of the Trust Securities (as defined in the Declaration). Notwithstanding
the foregoing, this Common Securities Guarantee will continue to be effective or
will be reinstated, as the case may be, if at any time any Holder of the Common
Securities must restore payment of any sums paid under the Common Securities or
under this Common Securities Guarantee.


                                    ARTICLE V
                                  MISCELLANEOUS

SECTION 5.1. SUCCESSORS AND ASSIGNS

     All guarantees and agreements contained in this Common Securities Guarantee
shall bind the successors, assigns, receivers, trustees and representatives of
the Guarantor and shall inure to the benefit of the Holders of the Common
Securities then outstanding.

SECTION 5.2.  AMENDMENTS

     Except with respect to any changes which do not adversely affect in any
material respect the rights of the Holders (in which case no consent of the
Holders will be required), this Common Securities Guarantee may only be amended
with the prior approval of the Holders of a majority in liquidation amount of
all the outstanding Common Securities. The provisions of Section

                                       7
<PAGE>   9

12.2 of the Declaration with respect to meetings of Holders of the Trust
Securities apply to the giving of such approval.

SECTION 5.3. NOTICES

     All notices provided for in this Common Securities Guarantee shall be in
writing, duly signed by the party giving such notice, and shall be delivered,
telecopied or mailed by registered or certified mail, as follows:

     (a)  if given to the Issuer, in care of the Administrative Trustee at the
Issuer's mailing address set forth below (or such other address as the Issuer
may give notice of to the Holders of the Common Securities):

               BFD Preferred Capital Trust II
               17 New England Executive Park
               Burlington, Massachusetts  01803
               Telephone: (781) 273-0300
               Telecopier: (781) 221-7594

     (b)  if given to the Guarantor, at the Guarantor's mailing address set
forth below (or such other address as the Guarantor may give notice of to the
Holders of the Common Securities):

               BostonFed Bancorp, Inc.
               17 New England Executive Park
               Burlington, Massachusetts  01803
               Telephone: (781) 273-0300
               Telecopier: (781) 221-7594

     (c)  if given to any Holder of the Common Securities, at the address set
forth on the books and records of the Issuer.

     All such notices shall be deemed to have been given when received in
person, telecopied with receipt confirmed, or mailed by first class mail,
postage prepaid except that if a notice or other document is refused delivery or
cannot be delivered because of a changed address of which no notice was given,
such notice or other document shall be deemed to have been delivered on the date
of such refusal or inability to deliver.

                                       8
<PAGE>   10

SECTION 5.4. BENEFIT

     This Common Securities Guarantee is solely for the benefit of the Holders
of the Common Securities and is not separately transferrable from the Common
Securities.

SECTION 5.5. GOVERNING LAW

     THIS COMMON SECURITIES GUARANTEE SHALL BE GOVERNED BY, AND CONSTRUED AND
INTERPRETED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK WITHOUT REGARD
TO CONFLICT OF LAW PRINCIPLES THEREOF.

     This Common Securities Guarantee is executed as of the day and year first
above written.

                                   BOSTONFED BANCORP, INC.


                                   By: /s/ John A. Simas
                                       -----------------------------------------
                                        John A. Simas
                                        Executive Vice President and
                                            Corporate Secretary


                                       9