<PAGE>   1
                                                                     EXHIBIT 4.8



                                     WARRANT

THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE SECURITIES
ACT OF 1933. SUCH SECURITIES AND ANY SECURITIES OR SHARES ISSUED HEREUNDER MAY
NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH REGISTRATION OR AN EXEMPTION
THEREFROM UNDER SAID ACT. COPIES OF THE AGREEMENT COVERING THE PURCHASE OF THESE
SECURITIES AND RESTRICTING THEIR TRANSFER OR SALE MAY BE OBTAINED AT NO COST BY
WRITTEN REQUEST MADE BY THE HOLDER OF RECORD HEREOF TO THE SECRETARY OF THE
COMPANY AT ITS PRINCIPAL EXECUTIVE OFFICES.

No. __ B                                          Warrant to Purchase __ Shares
                                            Common Stock (subject to adjustment)


                        WARRANT TO PURCHASE COMMON STOCK

                                       OF

                                 VIROLOGIC, INC.

                           VOID AFTER AUGUST 30, 2003

        This certifies that, for value received, ____________, or registered
assigns ("Holder") is entitled, subject to the terms set forth below, to
purchase from VIROLOGIC, INC. (the "Company"), a Delaware corporation, up to 57
shares of the Common Stock of the Company, as constituted on August 25, 1998
(the "Warrant Issue Date"), upon surrender hereof, at the principal office of
the Company referred to below, with the subscription form attached hereto duly
executed, and simultaneous payment therefor in lawful money of the United States
or otherwise as hereinafter provided, at the Exercise Price as set forth, in
Section 2 below. The number, character and Exercise Price of such shares of
Common Stock are subject to adjustment as provided below. The term "Warrant" as
used herein shall include this Warrant, which is one of a series of warrants
issued for the Common Stock of the Company, and any warrants delivered in
substitution or exchange therefor as provided herein.

        This Warrant is issued in connection with the transactions described in
that certain Series B Preferred Stock Purchase Agreement between the Company and
the investors described therein, dated as of August 25, 1998 (the "Purchase
Agreement").

        1. Term of Warrant. Subject to the terms and conditions set forth
herein, this warrant shall be exercisable, in whole or in part, during the term
commencing on the Warrant Issue Date and ending at 5:00 p.m., Pacific standard
time, on August 30, 2003, and shall be void thereafter.

        2. Exercise Price. The Exercise Price at which this Warrant may be
exercised shall be $0.01 per share of Common Stock, as adjusted from time to
time pursuant to Section 11 hereof.



                                       1.
<PAGE>   2

        3. Vesting of Warrant. Effective as of the Warrant Issue Date, this
Warrant is exercisable with respect to __ shares of Common Stock of the Company.

        4. Exercise of Warrant.

               (a) The purchase rights represented by this Warrant are
exercisable by the Holder in whole or in part, at any time, or from time to
time, during the term hereof as described in Section 1 above, by the surrender
of this Warrant and the Notice of Exercise annexed hereto duly completed and
executed on behalf of the Holder, at the office of the Company (or such other
office or agency of the Company as it may designate by notice in writing to the
Holder at the address of the Holder appearing on the books of the Company), upon
payment (i) in cash or by check acceptable to the Company, (ii) by cancellation
by the Holder of indebtedness of the Company to the Holder, or (iii) by a
combination of (i) and (ii), of the purchase price of the shares to be
purchased.

               (b) This Warrant shall be deemed to have been exercised
immediately prior to the close of business on the date of its surrender for
exercise as provided above, and the person entitled to receive the shares of
Common Stock issuable upon such exercise shall be treated for all purposes as
the holder of record of such shares as of the close of business on such date. As
promptly as practicable on or after such date and in any event within ten (10)
days thereafter, the Company at its expense shall issue and deliver to the
person or persons entitled to receive the same a certificate or certificates for
the number of shares issuable upon such exercise. In the event that this Warrant
is exercised in part, the Company at its expense will execute and deliver a new
Warrant of like tenor exercisable for the number of shares for which this
Warrant may then be exercised.

               (c) Notwithstanding any provisions herein to the contrary, if the
fair market value of one share of Common Stock is greater than the Exercise
Price (at the date of calculation as set forth below), in lieu of exercising
this Warrant for cash, the Holder may elect to receive shares equal to the value
(as determined below) of this Warrant (or the portion thereof being cancelled)
by surrender of this Warrant at the principal office of the Company together
with the properly endorsed Notice of Exercise and notice of such election in
which event the Company shall issue to the Holder a number of shares of Common
Stock computed using the following formula:

                                       Y (A - B)
                                  X =    -------
                                            A


Where   X   =   the number of shares of Common Stock to be issued to the Holder

        Y   =   the number of shares of Common Stock purchasable under the
                Warrant or, if only a portion of the Warrant is being exercised,
                the portion of the Warrant being canceled (at the date of such
                calculation)

        A   =   the fair market value of one share of the Company's Common Stock
                (at the date of such calculation)

        B   =   Exercise Price (as adjusted to the date of such calculation)



                                       2.
<PAGE>   3
For purposes of the above calculation, fair market value of one share of Common
Stock shall be determined by the Company's Board of Directors in good faith;
provided, however, that where there exists a public market for the Company's
Common Stock at the time of such exercise, the fair market value per share shall
be the average of the closing bid and asked prices of the Common Stock quoted in
the Over-The-Counter Market Summary or the last reported sale price of the
common Stock or the closing price quoted on The Nasdaq National Market or on any
exchange on which the Common Stock is listed, whichever is applicable, as
published in the Western Edition of The Wall Street Journal for the five (5)
trading days prior to the date of determination of fair market value.
Notwithstanding the foregoing, in the event the Warrant is exercised in
connection with the Company's initial public offering of Common Stock, the fair
market value per share shall be the per share offering price to the public of
the Company's initial public offering.

               (d) No fractional shares or scrip representing fractional shares
shall be issued upon the exercise of this Warrant. In lieu of any fractional
share to which the Holder would otherwise be entitled, the Company shall make a
cash payment equal to the Exercise Price multiplied by such fraction.

        5. Replacement of Warrant. On receipt 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, on delivery of an
indemnity agreement reasonably satisfactory in form and substance to the Company
or, in the case of mutilation, on surrender and cancellation of this Warrant,
the Company at its expense shall execute and deliver, in lieu of this Warrant, a
new warrant of like tenor and amount.

        6. Rights of Stockholders. Subject to Sections 9 and 11 of this Warrant,
the Holder shall not be entitled to vote or receive dividends or be deemed the
holder of the Common Stock to which this Warrant relates or any other securities
of the Company that may at any time be issuable on the exercise hereof for any
purpose.

        7. Transfer of Warrant.

               (a) Warrant Register. The Company will maintain a register (the
"Warrant Register") containing the names and addresses of the Holder or Holders.
Any Holder of this Warrant or any portion thereof may change his/her address as
shown on the Warrant Register by written notice to the Company requesting such
change. Any notice or written communication required or permitted to be given to
the Holder may be delivered or given by mail to such Holder as shown on the
Warrant Register and at the address shown on the Warrant Register. Until this
Warrant is transferred on the Warrant Register of the Company, the Company may
treat the Holder as shown on the Warrant Register as the absolute owner of this
Warrant for all purposes, notwithstanding any notice to the contrary.

               (b) Warrant Agent. The Company may, by written notice to the
Holder, appoint an agent for the purpose of maintaining the Warrant Register
referred to in Section 7(a) above, issuing the Common Stock or other securities
then issuable upon the exercise of this Warrant, exchanging this Warrant,
replacing this Warrant, or any or all of the foregoing.



                                       3.
<PAGE>   4
Thereafter, any such registration, issuance, exchange, or replacement, as the
case may be, shall be made at the office of such agent.

               (c) Transferability and Nonnegotiability of Warrant. This Warrant
may not be transferred or assigned (i) except in its entirety and (ii) without
compliance with all applicable federal and state securities laws by the
transferor and the transferee (including the delivery of investment
representation letters reasonably satisfactory to the Company, if such are
requested by the Company), and then only against receipt of an agreement of the
transferee to comply with the provisions of this Section 7(c) with respect to
any resale or other disposition of this Warrant. Subject to the provisions of
this warrant with respect to compliance with the Securities Act of 1933, as
amended (the "Act"), title to this Warrant may be transferred by endorsement (by
the Holder executing the Assignment Form annexed hereto) and delivery in the
same manner as a negotiable instrument transferable by endorsement and delivery.

               (d) Exchange of Warrant upon a Transfer. On surrender of this
Warrant for exchange, properly endorsed on the Assignment Form and subject to
the provisions of this warrant with respect to compliance with the Act and with
the limitations on assignments and transfers and contained in this Section 7,
the Company at its expense shall issue to or on the order of the Holder a new
warrant or warrants of like tenor, in the name of the Holder or as the Holder
(on payment by the Holder of any applicable transfer taxes) may direct, for the
number of shares issuable upon exercise hereof.

               (e) Compliance with Securities Laws.

                      (i) The Holder of this Warrant by acceptance hereof,
acknowledges that this Warrant and the shares of Common Stock to be issued upon
exercise hereof are being acquired solely for the Holder's own account and not
as a nominee for any other party, and for investment, and that the Holder will
not offer, sell or otherwise dispose of this Warrant or any shares of Common
Stock to be issued upon exercise hereof except under circumstances that will not
result in a violation of the Act or any state securities laws. Upon exercise of
this Warrant, the Holder shall, if requested by the Company, confirm in writing,
in a form satisfactory to the Company, that the shares of Common Stock so
purchased are being acquired solely for the Holder's own account and not as a
nominee for any other party, for investment, and not with a view toward
distribution or resale.

                      (ii) This Warrant and all shares of Common Stock issued
upon exercise hereof shall be stamped or imprinted with a legend in
substantially the following form (in addition to any legend required by state
securities laws):

        THE SECURITIES REPRESENTED HEREBY HAVE NOT BEEN REGISTERED UNDER THE
        SECURITIES ACT OF 1933. SUCH SECURITIES AND ANY SECURITIES OR SHARES
        ISSUED HEREUNDER MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF SUCH
        REGISTRATION OR AN EXEMPTION THEREFROM UNDER SAID ACT. COPIES OF THE
        AGREEMENT COVERING THE PURCHASE OF THESE SECURITIES AND RESTRICTING
        THEIR TRANSFER OR SALE MAY BE OBTAINED AT NO COST BY WRITTEN REQUEST
        MADE BY THE



                                       4.
<PAGE>   5
        HOLDER OF RECORD HEREOF TO THE SECRETARY OF THE COMPANY AT ITS PRINCIPAL
        EXECUTIVE OFFICES.

        8. Reservation of Stock. The Company covenants that during the term this
Warrant is exercisable, the Company will reserve from. its authorized and
unissued Common Stock a sufficient number of shares to provide for the issuance
of Common Stock upon the exercise of this warrant and, from time to time, will
take all steps necessary to amend its Certificate of Incorporation (the
"Certificate") to provide sufficient reserves of shares of Common Stock. The
Company further covenants that all shares that may be issued upon the exercise
of rights represented by this Warrant, upon exercise of the rights represented
by this Warrant and payment of the Exercise Price, all as set forth herein, will
be free from all taxes, liens and charges in respect of the issue thereof (other
than taxes in respect of any transfer occurring contemporaneously or otherwise
specified herein). The Company agrees that its issuance of this Warrant shall
constitute full authority to its officers who are charged with the duty of
executing stock certificates to execute and issue the necessary certificates for
shares of Common Stock upon the exercise of this Warrant.

        9. Notices.

               (a) Whenever the Exercise Price or number of shares purchasable
hereunder shall be adjusted pursuant to Section 11 hereof, the Company shall
issue a certificate signed by its Chief Financial Officer setting forth, in
reasonable detail, the event requiring the adjustment, the amount of the
adjustment the method by which such adjustment was calculated, and the Exercise
Price and number of shares purchasable hereunder after giving effect to such
adjustment, and shall cause a copy of such certificate to be mailed (by
first-class mail, postage prepaid) to the Holder of this Warrant.

               (b) In case:

                      (i) the Company shall take a record of the holders of its
Common Stock (or other stock or securities at the time receivable upon the
exercise or conversion of this Warrant) for the purpose of entitling them to
receive any dividend or other distribution, or any right to subscribe for or
purchase any shares of stock of any class or any other securities, or to receive
any other right, or

                      (ii) of any public offering of the Company's Common Stock,
any capital reorganization of the Company, any reclassification of the capital
stock of the Company, any consolidation or merger of the Company with or into
another corporation, or any conveyance of all or substantially all of the assets
of the Company to another corporation, or

                      (iii) of any voluntary dissolution, liquidation or
winding-up of the Company,

then, and in each such case, the Company will mail or cause to be mailed to the
Holder or Holders a notice specifying, as the case may be, (A) the date on which
a record is to be taken for the purpose of such dividend, distribution or right,
and stating the amount and character of such dividend, distribution or right, or
(B) the date on which such reorganization, reclassification, consolidation,
merger, conveyance, dissolution, liquidation or winding-up is to take place, and



                                       5.
<PAGE>   6
the time, if any is to be fixed, as of which the holders of record of Common
Stock (or such stock or securities at the time receivable upon the exercise of
this Warrant) shall be entitled to exchange their shares of Common Stock (or
such other stock or securities) for securities or other property deliverable
upon such reorganization, reclassification, consolidation, merger, conveyance,
dissolution, liquidation or winding-up. Such notice shall be mailed at least
fifteen (15) days prior to the date therein specified.

               (c) All such notices, advices and communications shall be deemed
to have been received (i) in the case of personal delivery, on the date of such
delivery and (ii) in the case of mailing, on the third business day following
the date of such mailing.

        10. Amendments. This Warrant and any term hereof may be changed, waived,
discharged or terminated only by an instrument in writing signed by the party
against which enforcement of such change, waiver, discharge or termination is
sought.

        11. Adjustments. The Exercise Price and the number of shares purchasable
hereunder are subject to adjustment from time to time as follows:

               11.1 Merger, Sale of Assets, etc.. If at any time while this
Warrant, or any portion thereof, is outstanding and unexpired there shall be (a)
a reorganization (other than a combination, reclassification, exchange or
subdivision of shares otherwise provided for herein), (b) a merger or
consolidation of the Company with or into another corporation in which the
Company is not the surviving entity, or a reverse triangular merger in which the
Company is the surviving entity but the shares of the Company's capital stock
outstanding immediately prior to the merger are converted by virtue of the
merger into other property, whether in the form of securities, cash or
otherwise, or (c) a sale or transfer of the Company's properties and assets as,
or substantially as, an entirety to any other person, then the Company shall
provide the holder of this Warrant with not less than thirty (30) days, prior
written notice of such event and an opportunity to exercise this Warrant
effective upon the consummation of such event, so that, as a part of such
reorganization, merger, consolidation, sale or transfer, the holder of this
Warrant shall be entitled to receive upon exercise of this Warrant the number of
shares of stock or other securities or property of the successor corporation
resulting from such reorganization, merger, consolidation, sale or transfer that
a holder of the shares deliverable upon exercise of this Warrant would have been
entitled to receive in such reorganization, consolidation, merger, sale or
transfer. The Company shall not effect any such consolidation, merger, sale,
transfer or other disposition unless prior to or simultaneously with the
consummation thereof the successor corporation (if other than the Company)
resulting from such consolidation or merger, or the corporation purchasing or
otherwise acquiring such assets or other appropriate corporation or entity shall
agree to deliver to the holder of the Warrant, upon its exercise and payment of
the Exercise Price then in effect, such shares of stock, securities or assets
as, in accordance with the foregoing provisions, such holder may be entitled to
purchase.

               11.2 Reclassification, etc.. If the Company, at any time while
this Warrant, or any portion thereof, remains outstanding and unexpired, by
reclassification of securities or otherwise, shall change any of the securities
as to which purchase rights under this Warrant exist into the same or a
different number of securities of any other class or classes, this Warrant shall
thereafter represent the right to acquire such number and kind of securities as
would have been



                                       6.
<PAGE>   7
issuable as the result of such change with respect to the securities that were
subject to the purchase rights under this Warrant immediately prior to such
reclassification or other change and the Exercise Price therefor shall be
appropriately adjusted, all subject to further adjustment as provided in this
Section 11.

               11.3 Split, Subdivision or Combination of Shares. If the Company
at any time while this Warrant, or any portion thereof, remains outstanding and
unexpired shall split, subdivide or combine the securities as to which purchase
rights under this warrant exist, into a different number of securities of the
same class, the Exercise Price for such securities shall be proportionately
decreased in the case of a split or subdivision or proportionately increased in
the case of a combination.

               11.4 Adjustments for Dividends in Stock or Other Securities or
Property. If while this Warrant, or any portion hereof, remains outstanding and
unexpired the holders of the securities as to which purchase rights under this
Warrant exist at the time shall have received, or, on or after the record date
fixed for the determination of eligible Stockholders, shall have become entitled
to receive, without payment therefor, other or additional stock or other
securities or property (other than cash) of the Company by way of dividend, then
and in each case, this Warrant shall represent the right to acquire, in addition
to the number of shares of the security receivable upon exercise of this
Warrant, and without payment of any additional consideration therefor, the
amount of such other or additional stock or other securities or property (other
than cash) of the Company that such holder would hold on the date of such
exercise had it been the holder of record of the security receivable upon
exercise of this Warrant on the date hereof and had thereafter, during the
period from the date hereof to and including the date of such exercise, retained
such shares and/or all other additional stock available by it as aforesaid
during such period, giving effect to all adjustments called for during such
period by the provisions of this Section 11.

               11.5 Certificate as to Adjustments. Upon the occurrence of each
adjustment or readjustment pursuant to this Section 11, the Company at its
expense shall promptly compute such adjustment or readjustment in accordance
with the terms hereof and furnish to each Holder of this Warrant a certificate
setting forth such adjustment or readjustment and showing in detail the facts
upon which such adjustment or readjustment is based. The Company shall, upon the
written request, at any time, of any such Holder, furnish or cause to be
furnished to such Holder a like certificate setting forth: (a) such adjustments
and readjustments; (b) the Exercise Price at the time in effect; and (c) the
number of shares and the amount, if any, of other property that at the time
would be received upon the exercise of the Warrant.

               11.6 No Impairment. The Company will not, by any voluntary
action, avoid or seek to avoid the observance or performance of any of the terms
to be observed or performed hereunder by the Company, but will at all times in
good faith assist in the carrying out of ail the provisions of this Section 11
and in the taking of all such action as may be necessary or appropriate in order
to protect the right of the Holders of this Warrant against impairment.

        12. Registration Rights. Upon exercise of this Warrant, the Holder shall
have and be entitled to exercise, together with all other holders of Registrable
Securities possessing registration rights under that certain Investors' Rights
Agreement, of even date herewith, between



                                       7.
<PAGE>   8
the Company and the parties who have executed the counterpart signature pages
thereto or are otherwise bound thereby (the "Investors' Rights Agreement"), the
rights of registration granted under the Investors' Rights Agreement to
Registrable Securities (with respect to the Shares issued on exercise of this
Warrant). By its receipt of this Warrant, Holder agrees to be bound by the
Investors' Rights Agreement upon exercise of this warrant as a party thereto.

        13. Miscellaneous.

               (a) Successors. All the covenants and provisions hereof by or for
the benefit of the Company or the Holder shall bind and inure to the benefit of
their respective successors and assigns hereunder.

               (b) Governing Law. This Warrant shall be deemed to be a contract
made under the laws of the State of California and for all purposes shall be
construed in accordance with the laws of said State.

               (c) Attorneys Fees in the Event of a Dispute. In the event of any
action at law, suit in equity or arbitration proceeding in relation to this
Warrant or any Common Stock issued or to be issued hereunder, the prevailing
party or parties, shall be paid by the other party or parties a reasonable sum
for attorneys, fees and expenses of such prevailing party or parties.

               (d) Saturdays, Sundays, Holidays. If the last or appointed day
for the taking of any action or the expiration of any right required or granted
herein shall be a Saturday or a Sunday or shall be a legal holiday in the State
of California, then such action may be taken or such right may be exercised on
the next succeeding day not a legal holiday.

        IN WITNESS WHEREOF, VIROLOGIC, INC. has caused this Warrant to be
executed by its officers thereunto duly authorized.


        Dated: August 25, 1998.

                                       VIROLOGIC, INC.



                                       By
                                         ---------------------------------------
                                                       President and CEO



                                       8.
<PAGE>   9
                               NOTICE OF EXERCISE



To: VIROLOGIC, INC.

        (1) The undersigned hereby elects to purchase shares of Common Stock of
VIROLOGIC, INC., pursuant to the terms of the attached Warrant, and tenders
herewith payment of the purchase price for such shares in full.

        (2) In exercising this Warrant, the undersigned hereby confirms and
acknowledges that the shares of Common Stock are being acquired solely for the
account of the undersigned and not as a nominee for any other party, or for
investment,. and that the undersigned will not offer, sell or otherwise dispose
of any such shares of Common Stock except under circumstances that will not
result in a violation of the Securities Act of 1933, as amended, or any state
securities laws.

        (3) Please issue a certificate or certificates representing said shares
of Common Stock in the name of the undersigned or in such other name as is
specified below:



                                       -----------------------------------------
                                                        (Name)



                                       -----------------------------------------
                                                        (Name)


        (4) Please issue a new Warrant for the unexercised portion of the
attached Warrant in the name of the undersigned or in such other name as is
specified below:



                                       -----------------------------------------
                                                        (Name)


-----------------------------          -----------------------------------------
          (Date)                                     (Signature)



                                       9.
<PAGE>   10
                                 ASSIGNMENT FORM

        FOR VALUE RECEIVED, the undersigned registered owner of this Warrant
hereby sells, assigns and transfers unto the Assignee named below all of the
rights of the undersigned under the within Warrant, with respect to the number
of shares of Common Stock set forth below:

<TABLE>
<CAPTION>
   Name of Assignee                 Address                       No. of Shares
   ----------------                 -------                       -------------
<S>                                 <C>                           <C>
</TABLE>



and does hereby irrevocably constitute and appoint Attorney to make such
transfer on the books of VIROLOGIC, INC., maintained for the purpose, with full
power of substitution in the premises.

        The undersigned also represents that by assignment hereof, the Assignee
acknowledges that this warrant and the shares of stock to be issued upon
exercise hereof or conversion thereof are being acquired for investment and that
the Assignee will not offer, sell or otherwise dispose of this warrant or any
shares of stock to be issued upon exercise hereof or conversion thereof except
under circumstances which will not result in a violation of the Securities Act
of 1933, as amended, or any state securities laws. Further, the Assignee has
acknowledged that upon exercise of this Warrant, the Assignee shall, if
requested by the Company, confirm in writing, in a form satisfactory to the
Company, that the shares of stock so purchased are being acquired for investment
and not with a view toward distribution or resale.

        Dated:


                                      ------------------------------------------
                                                 Signature of Holder



                                      10.