<PAGE>   1

                                                                       EXHIBIT 4

COMMON STOCK                                               COMMON STOCK

         NUMBER                                                SHARES
          SC

                             [S1 LOGO APPEARS HERE]

                                 S1 CORPORATION

INCORPORATED UNDER THE LAWS                               SEE REVERSE SIDE FOR
OF THE STATE OF DELAWARE                                  CERTAIN LEGENDS

                                                          CUSIP 78463B 10 1

This Certifies that

is the owner of

FULLY PAID AND NONASSESSABLE SHARES OF COMMON STOCK, PAR VALUE $0.01 PER SHARE,
                                       OF

S1 Corporation (the "Corporation"), a Delaware corporation with its principal
executive office located in Atlanta, Georgia. The shares represented by this
certificate are transferable only on the stock transfer books of the Corporation
by the holder of record hereof, or by his or her duly authorized attorney or
legal representative, upon the surrender of this certificate properly endorsed.

     This certificate is not valid unless countersigned and registered by the
Transfer Agent and Registrar.

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

Dated:

                                 S1 CORPORATION

                                     [SEAL]


                                      By:
/s/ Nancy K. Kenley                       /s/ James S. Mahan, III
       SECRETARY                                 PRESIDENT

COUNTERSIGNED AND REGISTERED:

  AMERICAN STOCK TRANSFER & TRUST COMPANY
                      (NEW YORK)      TRANSFER AGENT
                                      AND REGISTRAR

BY

                               AUTHORIZED SIGNATURE


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                                 S1 CORPORATION

     The shares represented by this certificate are issued subject to all the
provisions of the certificate of incorporation and bylaws of S1 Corporation (the
"Corporation") as from time to time amended (copies of which are on file at the
principal executive office of the Corporation), to all of which the holder by
acceptance hereof assents.

     The Corporation is authorized to issue more than one class or series of
stock. The Corporation will furnish to any shareholder, upon request and without
charge, a full statement of the powers, designations, preferences and relative,
participating, optional, or other special rights of each authorized class of
stock or series thereof and the qualifications, limitations or restrictions of
such preferences and/or rights, to the extent that the same have been fixed, and
of the authority of the board of directors to designate the same with respect to
other series. Such request may be made to the Corporation at its principal
executive office.

     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:

<TABLE>
<S>                                                         <C>
         TEN COM    - as tenants in common                    UNIF GIFT MIN ACT-__________Custodian__________
         TEN ENT    - as tenants by the entireties                               (Cust)             (Minor)
         JT TEN     - as joint tenants with right of                            under Uniform Gifts to Minors
                      survivorship and not as tenants                           Act_______________
                      in common                                                       (State)
</TABLE>

    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 TYPE NAME AND ADDRESS, INCLUDING ZIP CODE, OF ASSIGNEE)

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

Dated
      ------------------

                                   --------------------------------------------
                    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 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.