<PAGE>
                                                                     Exhibit 4.1


                         [SPECIMEN OF STOCK CERTIFICATE]

                           [FRONT SIDE OF CERTIFICATE]

                         BENJAMIN FRANKLIN BANCORP, INC.

        INCORPORATED UNDER THE LAWS OF THE COMMONWEALTH OF MASSACHUSETTS

[NUMBER]                                            [SHARES]

BFBC *XX*                                            * XX *

COMMON STOCK                           SEE REVERSE FOR CERTAIN RESTRICTIONS
NO PAR VALUE                       AND DEFINITIONS AND FOR CERTAIN PREFERENCES
                                WHICH MAY EXIST WITH RESPECT TO THE COMMON STOCK

________________________________________________________________________________
                  This Certifies that SPECIMEN CUSIP
                  082073 10 7



                  is the owner of NO _ _ _ _ _ _ __
________________________________________________________________________________

            FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK,

                           NO PAR VALUE PER SHARE, OF

                         BENJAMIN FRANKLIN BANCORP, INC.

Transferable on the books of the Corporation by the holder hereof in person or
by duly authorized attorney upon surrender of this Certificate properly endorsed
or assigned. This Certificate and the shares of common stock represented hereby
are issued and shall be held subject to the laws of The Commonwealth of
Massachusetts and to the Articles of Organization and By-Laws of the
Corporation, as in effect and as amended from time to time hereafter. This
Certificate is not valid until countersigned and registered by the Transfer
Agent and Registrar.

      IN WITNESS WHEREOF, the Corporation has caused its facsimile corporate
seal and facsimile signatures of its duly authorized officers to be hereunto
affixed.

                         BENJAMIN FRANKLIN BANCORP, INC.

/s/________________________________    SEAL  /s/________________________________
   TREASURER                                    PRESIDENT


COUNTERSIGNED AND REGISTERED:

AMERICAN STOCK TRANSFER &
TRUST COMPANY

(New York, NY)

TRANSFER AGENT AND REGISTRAR


BY____________________________

   AUTHORIZED SIGNATURE
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                         [BACK SIDE OF CERTIFICATE]

      The shares represented by this Certificate are issued subject to all the
provisions of the Articles of Organization and By-Laws of the Corporation, as in
effect and as amended from time to time, to all of which the holder by
acceptance hereof assents. The Corporation will furnish to any shareholder, upon
written request and without charge, a copy of the Articles of Organization and
By-Laws of the Corporation. Such request may be made to the Clerk of the
Corporation.

      Under the Articles of Organization, the Board of Directors may, without
stockholder approval but subject to certain regulatory approvals, reclassify any
unissued shares of common stock into one or more series of preferred stock and
designate and issue one or more series of preferred stock, establish the number
of shares in each such series, fix and state the voting powers, designations,
preferences and the relative or special rights or privileges of the shares of
any series so established and the qualifications thereon without further vote or
action by the stockholders. A statement of the preferences, powers,
qualifications and rights of the series and classes of preferred stock will be
furnished to the holder of this Certificate upon written request and without
charge.

      The following abbreviations, when used in the inscription on the face of
this Certificate, shall be construed as though they were written out in full
according to applicable laws or regulations:

TEN COM --  as tenants in common       UNIF GIFT MIN ACT-- Custodian ...(Cust)
                                                           (Minor) under Uniform
                                                           Gifts to Minors Act
                                                           (State)
TEN ENT --  as tenants by the entireties
JT TEN --   as joint tenants with right
            of survivorship and not as
            tenants in common

      Additional abbreviations may also be used though not in the above list.

      For value received, __________________ hereby sell, assign and transfer
      unto

PLEASE INSERT SOCIAL SECURITY OR OTHER
IDENTIFYING NUMBER OF ASSIGNEE

________________________________________________________________________________


________________________________________________________________________________
(PLEASE PRINT OR TYPEWRITE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

________________________________________________________________ shares of the
common stock represented by the within Certificate, and do hereby irrevocably
constitute and appoint _________________________________________ Attorney to
transfer the said stock on the books of the within named Corporation with full
power of substitution in the premises.

Dated: _________________________


           NOTICE:            __________________________________________________
                              THE SIGNATURE OF THIS ASSIGNMENT MUST CORRESPOND
                              WITH THE NAME AS WRITTEN UPON THE FACE OF THE
                              CERTIFICATE IN EVERY PARTICULAR, WITHOUT
                              ALTERATIONS OR ENLARGEMENT OR ANY CHANGE WHATEVER.

SIGNATURE(S) GUARANTEED:      __________________________________________________
                              THE SIGNATURE(S) SHOULD BE GUARANTEED BY AN
                              ELIGIBLE GUARANTOR INSTITUTION (BANKS,
                              STOCKBROKERS, SAVINGS AND LOAN ASSOCIATIONS AND
                              CREDIT UNIONS WITH MEMBERSHIP IN AN APPROVED
                              SIGNATURE GUARANTEE MEDALLION PROGRAM), PURSUANT
                              TO S.E.C. RULE 17AD-15.

NOTICE: The signature to this assignment must correspond with the name as
        written upon the face of the Certificate, in every particular, without
        alteration or enlargement, or any change whatsoever.