Document:

<PAGE>
                                                                     Exhibit 4.1

COMMON STOCK                                                        COMMON STOCK
   NUMBER                                                              SHARES
MEM

                          MEMORY PHARMACEUTICALS CORP.
              INCORPORATED UNDER THE LAWS OF THE STATE OF DELAWARE

  SEE REVERSE FOR
CERTAIN DEFINITIONS
                                                               CUSIP 58606R 40 3
THIS IS TO CERTIFY THAT

is the owner of

 FULLY PAID AND NON-ASSESSABLE SHARES OF THE COMMON STOCK, PAR VALUE $0.001 PER
                                   SHARE, OF

                          MEMORY PHARMACEUTICALS CORP.

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. This certificate and the shares represented hereby are issued and
shall be held subject to all of the provisions of the Restated Certificate of
Incorporation of the Corporation and all amendments thereof to all of which the
holder by the acceptance hereof assents. This certificate is not valid unless
countersigned and registered by the Transfer Agent and Registrar.

     WITNESS the facsimile seal of the Corporation and the facsimile signatures
of its duly authorized officers.

Dated

/s/ Dennis M. Keane                                        /s/ Axel J. Unterbeck
          SECRETARY                                                    PRESIDENT

                          [MEMORY PHARMACEUTICALS CORP.
                                 CORPORATE SEAL
                                      2000
                                   DELAWARE]

COUNTERSIGNED AND REGISTERED:
     AMERICAN STOCK TRANSFER & TRUST COMPANY
                    (NEW YORK, N.Y.)                       TRANSFER AGENT
                                                            AND REGISTRAR

BY
                                     AUTHORIZED SIGNATURE
<PAGE>

                          MEMORY PHARMACEUTICALS CORP.

      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)
TEN ENT  -- as tenants by the                         under Uniform Gifts to
            entireties                                Minors
                                                      Act
JT TEN   -- as joint tenants                             ---------------------
            with right of                                      (State)
            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 Common Stock represented by the within Certificate, and do hereby
irrevocably constitute and appoint

----------------------------------------------------------------------- Attorney
to transfer the said shares of Common Stock on the books of the Corporation
with full power of substitution in the premises.

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

                     -----------------------------------------------------------
                     NOTICE: THE SIGNATURE(S) TO THIS ASSIGNMENT MUST CORRESPOND
                             WITH THE NAME(S) 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) MUST 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.

KEEP THIS CERTIFICATE IN A SAFE PLACE. IF IT IS LOST, STOLEN, MUTILATED OR
DESTROYED, THE CORPORATION WILL REQUIRE A BOND OF INDEMNITY AS A CONDITION TO
THE ISSUANCE OF A REPLACEMENT CERTIFICATE.<PAGE>
                                                                     Exhibit 4.7

                                   Form of
      Amendment to the Investor Rights Agreement dated as of February 27, 2004
      (this "Amendment"), among Memory Pharmaceuticals Corp. and the investors
      set forth on the signature page hereto (the "Investors")

            A. The Company and the Investors entered into a Fourth Amended and
Restated Investor Rights Agreement dated as of September 11, 2003, as amended by
an Agreement dated as of December 18, 2003, among the Company, Dr. Eric Kandel
and the Investors (the "Investor Rights Agreement"). Capitalized terms used and
not otherwise defined herein shall have the meanings ascribed to such terms in
the Investor Rights Agreement.

            B. The Company and the Investors wish to amend the definition of
"Initial Public Offering" in the Investor Rights Agreement.

            C. Pursuant to Section 8.3 of the Investor Rights Agreement, this
Amendment requires the Company's consent and the consent in writing of holders
of at least two-thirds (2/3) of the outstanding shares of Preferred Stock.

            The Company and the Investors hereby agree as follows:

            1. Section 1.3 of the Investor Rights Agreement is hereby amended to
delete the definition of "Initial Public Offering" in its entirety and replace
it with the following:

            "Initial Public Offering" shall mean the closing of a firm
            commitment underwritten public offering pursuant to an effective
            registration statement under the Securities Act of 1933, as amended,
            covering the offer and sale of Common Stock for the account of the
            Company to the public at an initial public offering price per share
            of not less than $7.50 (such dollar amount being subject to
            equitable adjustment in the event of any stock dividend, stock
            split, combination, reorganization, recapitalization or similar
            event involving a change in the Common Stock) with net proceeds to
            the Company of not less than $25,000,000."

            2. Except as specifically amended by this Amendment, the Investor
Rights Agreement remains in full force and effect. Wherever the terms of this
Amendment and the Investor Rights Agreement conflict, this Amendment controls.
All references in the Investor Rights Agreement to "this Agreement" shall mean
the Investor Rights Agreement, as amended hereby.

            3. This Agreement may be executed in counterparts, which together
shall constitute one and the same instrument.
<PAGE>
      In witness whereof, the undersigned have executed this Amendment to the
Investor Rights Agreement as of the date first above written.

Company:                               Memory Pharmaceuticals Corp.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

Investors:                             Oxford Bioscience Partners II L.P.

                                       By: OBP Management II L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Oxford Bioscience Partners (Bermuda) II
                                       Limited Partnership

                                       By: OBP Management II (Bermuda) Limited
                                       Partnership

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                      Oxford Bioscience Partners (Adjunct) II,
                                      L.P.

                                      By: OBP Management II L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       Oxford Bioscience Partners (Gs-Adjunct)
                                       II, L.P.

                                       By: OBP Management II L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Healthcare Ventures V L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Hudson Trust

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Venrock Associates

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Venrock Associates II, L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       Alta Embarcadero Partners II LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       GLSLP Investment I Ltd.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       S.R. One Limited

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Alexandria Real Estate Equities, L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Pharma/w Health

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       Finsbury Worldwide Pharmaceutical Trust

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Eaton Vance Worldwide Health Sciences
                                       Portfolio

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Artal Services N.V.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Venrock Entrepreneuers Fund, L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Medica II Investment (Israel), LP

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       Medica II Investment (International), LP

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                      GIMV

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Memories Plus, LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       M&G Equities

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       -----------------------------------------
                                       Kenneth J. Novack

                                       -----------------------------------------
                                       Larry Abrams

<PAGE>
                                       Hare & Co., FAO Finsbury Worldwide
                                       Pharmaceutical Trust PLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Adviesbeheer Gimv Life Sciences

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Bioveda Fund Pte Ltd

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       By:Bioveda Capital Pte Ltd, Its
                                          Investment Manager

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Medica II Investments (P.F.)(Israel) L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                      Oxford Bioscience Partners II (Annex)
                                      L.P.

                                       By:  OBP Management II L.P.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       The Yasuda Enterprise Development I
                                       Limited Partnership

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Gray Ghost, LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Mitsubishi Corporation

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Mitsubishi International Corporation

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       CZ Venture Operations, Inc.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Kenneth A. Sorensen, Ph.D.

                                       Hans-Jurgen Hess, Ph.D.

                                       Mchugh, Divincent, Allessi, Inc.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       SG Cowen Ventures II, LP

                                       By: Societe Generale Investment
                                           Corporation, Its General Partner

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       Alexander Scriabine, M.D.
                                       RHO Management Trust II

                                       By:  Rho Capital Partners, Inc.,
                                            Investment Advisor

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Yamanouchi Venture Capital, LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Alta California Partners II, L.P.

                                       By:  Alta California Management Partners
                                       II, LLC, Its General Partner

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Biomedicine L.P.

                                       By: International Bm Biomedicine
                                           Holdings (Cayman) Ltd., Its General
                                           Partner

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:
<PAGE>
                                       New England Partners Capital, L.P.

                                       By:  Nep Capital, Llc, Its General
                                       Partner

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       Steven Ostrofsky Middlegate Ventures, LLC

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

                                       -----------------------------------------
                                       David I. Wolsk

                                       -----------------------------------------
                                       David R. Novack

                                       -----------------------------------------
                                       Helen Novack

                                       -----------------------------------------
                                       Hyman Novack

                                       -----------------------------------------
                                       Natalie C. Scriabine

                                       -----------------------------------------
                                       Christine B. Scriabine
<PAGE>
                                       -----------------------------------------
                                       Peter Smith

                                       -----------------------------------------
                                       Judith Smith

                                       -----------------------------------------
                                       Stephen White

                                       -----------------------------------------
                                       Mariam White

                                       Hoffmann-La Roche Inc.

                                       By:
                                          --------------------------------------
                                          Name:
                                          Title:

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