Document:

<PAGE>   1

                                                                     EXHIBIT 4.1

NUMBER                                                                    SHARES

                                 [AVISTAR LOGO]

INCORPORATED UNDER THE LAWS                     THIS CERTIFICATE IS TRANSFERABLE
 OF THE STATE OF DELAWARE                         IN CANTON, MA OR NEW YORK, NY

                                            SEE REVERSE FOR CERTAIN DEFINITIONS
                                                      CUSIP 05379X 10 9

This Certifies that:

is the record holder of

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

                       AVISTAR COMMUNICATIONS CORPORATION

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 is not valid until countersigned by the Transfer
Agent and registered by the Registrar.

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

     Dated:

               [AVISTAR COMMUNICATIONS CORPORATION CORPORATE SEAL
                           MARCH 23, 2000, DELAWARE]

/s/ [Signature Illegible]                              /s/ G. J. BURNETT

CHIEF FINANCIAL OFFICER AND SECRETARY                  PRESIDENT

COUNTERSIGNED AND REGISTERED:

     EQUISERVE TRUST COMPANY, N.A.

     TRANSFER AGENT AND REGISTRAR

BY  /s/ [Signature Illegible]

    AUTHORIZED SIGNATURE

<PAGE>   2
        The Corporation shall furnish without charge to each stockholder who so
requests a statement of the powers, designations, preferences and relative,
participating, optional, or other special rights of each class of stock of the
Corporation or series thereof and the qualifications, limitations or
restrictions of such preferences and/or rights. Such requests shall be made to
the Corporation's Secretary at the principal office of the Corporation.

        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)
                                                        UNIF TRF MIN ACT  -- .............Custodian (until age.........)
                                                                                 (Cust)
                                                                             ....................under Uniform Transfers
                                                                                    (Minor)
                                                                             to Minors Act..............................
                                                                                                   (State)
</TABLE>

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

        For Value Received,___________________________hereby sell(s), assign(s)
and transfer(s) 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________________________

                                X_______________________________________________

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

Signature(s) Guaranteed

By_____________________________________
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.

                           AMERICAN BANK NOTE COMPANY
                             55TH AND SANSOM STREET
                             PHILADELPHIA, PA 19139
                                 (215) 764-8600
                         SALES: M. SANDHU: 415-543-8585
                       /HOME 14/LIVE JOBS/A/AVISTAR 66908

              PRODUCTION COORDINATOR: MARY TARTAGLAI 215-764-8621
                             PROOF OF JUNE 27, 2000
                       AVISTAR COMMUNICATIONS CORPORATION
                                   H 66908 BK
                                  OPERATOR: JW
                                      NEW<PAGE>   1
                                                                  EXHIBIT 10.8.1

                                             Confidential Treatment Requested by
                                              Avistar Communications Corporation

                MODIFICATION #5 TO PREFERRED SUPPLIER AGREEMENT
                       DATED JUNE 24, 1997 ("AGREEMENT").
       Effective Date of Modification June 27, 2000 ("Modification Date").

The parties hereby agree that as of the Modification Date, certain terms and
conditions of the Agreement, as expressly modified below, shall apply and that
this Modification #5 shall take precedence over the terms contained in the
Agreement and any prior modifications:

1.   NEW ORDER: In conjunction with execution of this Modification #5, an
     Avistar purchase order will be placed for an additional * (or, as
     applicable pursuant to Section 8, * Codec units) ("New Order"). The unit
     price for each Codec on the New Order shall be * plus a per unit
     maintenance fee of *, for a total of purchase price per unit of *. The per
     unit maintenance fee shall replace the annual maintenance fee set forth in
     the Agreement, Section 3.4(b).

2.   PAYMENT TERMS FOR NEW ORDER: Payment terms extended to Avistar for the New
     Order shall be strictly adhered to. Payment terms for all deliveries under
     the New Order shall be * from shipment and Avistar's receipt of invoice.

3.   DELIVERY OF NEW ORDER: Codec units will be shipped in accordance with the
     following estimated delivery schedule, as updated from time to time by
     Avistar. Unless otherwise mutually agreed in writing, Tandberg will ship
     all Codec units under this New Order by no later than *:

<TABLE>
<CAPTION>
# OF UNITS OF THE * TO BE SHIPPED         EXT. SHIP DATE
---------------------------------         --------------
<S>                                      <C>
*                                         *

</TABLE>

4.   MAINTENANCE AND WARRANTY FOR NEW ORDER: All Codec units delivered pursuant
     to this Modification #5 shall be subject to a * warranty and maintenance
     support period from shipment ("Term"). Maintenance services shall be as
     described in the Agreement, including all software upgrades (not deemed to
     be products in their own right) issued during the Term.

5.   ADDITIONAL ORDERS FOR MODEL * CODECS: Avistar acknowledges that Tandberg
     wishes to cease normal production cycles for the Tandberg * Codec and has
     provided Avistar a proposal for an enhanced replacement version as further
     described in Section 6 below. The parties agree that in the event Avistar
     wishes to continue purchasing additional * Codec units, Tandberg will
     manufacture them for Avistar until the end of the *, at the prices
     indicated in Section 1 above, provided that Avistar provides Tandberg with
     (i) a * lead-time from order placement to delivery; and (ii) a minimum
     order of * units.

6.   ENHANCED REPLACEMENT *: Tandberg has suggested it's * as alternative
     replacement Codecs to Avistar in place of the * codecs. The * will be
     available for delivery by no later than *. The functionality of the * shall
     have no less than the functionality and features of the * Codec at a price
     not to exceed * (including maintenance). A * will also be available to
     Avistar at a price not to exceed * (including maintenance). The parties
     will negotiate in good faith the prices for the Enhanced Codecs beginning
     in * based on order quantity, etc.

7.   TESTING UNITS: To facilitate the testing and development effort required to
     ensure Avistar's ability to use the * units proposed by Tandberg, Tandberg
     will, at Avistar's request, provide Avistar up to * loaner units of each
     model of Codec for development and testing purposes. These test units will
     either become part of the first order for * units, or will be returned to
     Tandberg by *. In the case of * units, the parties may mutually agree to
     (i) an extension to the loaner term for these units if Avistar is required
     to provide support for the * (ii) Avistar's return of * of it's existing
     * units in exchange for the * units.

8.   INTERIM SUBSTITUTION OF * CODEC: Tandberg may, at its discretion, ship *
     Codecs to fulfill this Order provided that (i) the * software is installed
     to ensure compatibility with the Avistar System and that the Codec is
     'packaged in such a way so as not to expressly indicate it is not a * unit.
     In the event *, Avistar will ensure that the * capabilities are not
     utilized without first paying Tandberg a * upgrade fee.

9.   ADDITIONAL PRODUCTS, PACKAGING AND SHIP-TO LOCATIONS: At Avistar's request,
     Tandberg agrees to package the Codec in a bundle to include certain
     additional hardware and cables consistent with the Bill-of-Material options
     attached hereto as Exhibit 1 ("Avistar BOM"). Certain Avistar BOM options
     include * units at * per unit. The prices for the additional components of
     the Avistar BOM are detailed on Exhibit 1. When ordered separately from
     Codec units, the * units must be ordered in minimum increments of * units.
     At no additional charge to Avistar, bundled shipments in minimum increments
     of * units, will be sent to one of the following two Avistar Locations, as
     specified by Avistar:

          US:   992 SAN ANTONIO ROAD, PALO ALTO -- CA
          ---------------------------------------------------------------
          UK:   TO BE PROVIDED
          ---------------------------------------------------------------

Except as expressly modified by this Modification #5, all terms and conditions
of the Agreement shall remain in full force and effect until *. The parties
agree that this Modification #5 may be executed via facsimile and that it shall
be considered equivalent to an executed original when signed (via facsimile) by
both parties; nevertheless, any one of the parties may require the follow-up
exchange of originals in hardcopy by so requesting in writing within five (5)
days of facsimile execution.

IN WITNESS WHEREOF, THE PARTIES HAVE CAUSED THIS MODIFICATION #5 TO BE EXECUTED
BY THEIR AUTHORIZED REPRESENTATIVES:

AVISTAR SYSTEMS CORPORATION                                        TANDBERG INC.

By:
   --------------------------------------    -----------------------------------
Printed Name/Title: Rima Vanhill,            Printed Name/Title:
                    Director of Contracts

--------------
* Confidential portions omitted pursuant to a confidential treatment request
and filed separately with the Securities and Exchange Commission.

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