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                                                                    EXHIBIT 99.4

                                                                    Acceleration

                               CISCO SYSTEMS, INC.

                        STOCK OPTION ASSUMPTION AGREEMENT
                                  CALISTA, INC.
                         1999 EMPLOYEE STOCK OPTION PLAN

OPTIONEE:  <<First_Name>> <<Last_Name>>,

        STOCK OPTION ASSUMPTION AGREEMENT effective as of the 12th day of
November, 1999 by Cisco Systems, Inc., a California corporation ("Cisco").

        WHEREAS, the undersigned individual ("Optionee") holds one or more
outstanding options to purchase shares of the common stock of Calista, Inc., a
Delaware corporation ("Calista"), which were granted to Optionee under the
Calista, Inc. 1999 Employee Stock Option Plan (the "Plan") and are each
evidenced by a Notice of Stock Option Award and a Stock Option Award Agreement
(collectively, the "Option Agreement").

        WHEREAS, Calista has been acquired by Cisco through the merger of
Calista with and into Cisco (the "Merger") pursuant to the Agreement and Plan of
Reorganization, by and between Cisco and Calista (the "Merger Agreement").

        WHEREAS, the provisions of the Merger Agreement require Cisco to assume
all obligations of Calista under all outstanding options under the Plan at the
consummation of the Merger and to issue to the holder of each outstanding option
an agreement evidencing the assumption of such option.

        WHEREAS, pursuant to the provisions of the Merger Agreement, the
exchange ratio (the "Exchange Ratio") in effect for the Merger is 0.052774 of a
share of Cisco common stock ("Cisco Stock") for each outstanding share of
Calista common stock ("Calista Stock").

        WHEREAS, this Agreement became effective immediately upon the
consummation of the Merger (the "Effective Time") in order to reflect certain
adjustments to Optionee's outstanding options which have become necessary by
reason of the assumption of those options by Cisco in connection with the
Merger.

        NOW, THEREFORE, it is hereby agreed as follows:

        1. The number of shares of Calista Stock subject to the options held by
Optionee immediately prior to the Effective Time (the "Calista Options") and the
exercise price payable per share are set forth below. Cisco hereby assumes, as
of the Effective Time, all the duties and obligations of Calista under each of
the Calista Options. In connection with such assumption, the number of shares of
Cisco Stock purchasable under each Calista Option hereby assumed and the
exercise price payable thereunder have been adjusted to reflect the Exchange
Ratio. Accordingly, the number of shares of Cisco Stock subject to each Calista
Option hereby assumed shall be as specified for that option below, and the
adjusted exercise price payable per


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share of Cisco Stock under the assumed Calista Option shall also be as indicated
for that option below.

<TABLE>
<CAPTION>
             CALISTA STOCK OPTIONS                            CISCO ASSUMED OPTIONS
  -------------------------------------------     -------------------------------------------
  # of Shares of Calista       Exercise Price     # of Shares of Cisco      Adjusted Exercise
       Common Stock              per Share            Common Stock           Price per Share
  ----------------------       --------------     --------------------      -----------------
 <S>                           <C>                <C>                       <C>
      Calista Shares           $Calista Price        <<Cisco_Shares>>        $<<Cisco_Price>>
</TABLE>

        2. The intent of the foregoing adjustments to each assumed Calista
Option is to assure that the spread between the aggregate fair market value of
the shares of Cisco Stock purchasable under each such option and the aggregate
exercise price as adjusted pursuant to this Agreement will, immediately after
the consummation of the Merger, be not less than the spread which existed,
immediately prior to the Merger, between the then aggregate fair market value of
the Calista Stock subject to the Calista Option and the aggregate exercise price
in effect at such time under the Option Agreement. Such adjustments are also
intended to preserve, immediately after the Merger, on a per share basis, the
same ratio of exercise price per option share to fair market value per share
which existed under the Calista Option immediately prior to the Merger.

        3. The following provisions shall govern each Calista Option hereby
assumed by Cisco:

                (a) Unless the context otherwise requires, all references in
        each Option Agreement and, if applicable, in the Plan (as incorporated
        into such Option Agreement) (i) to the "Company" shall mean Cisco, (ii)
        to "Common Stock", "Stock" or "Share" shall mean a share of Cisco Stock,
        (iii) to the "Board" shall mean the Board of Directors of Cisco and (iv)
        to the "Committee" shall mean the Compensation Committee of the Cisco
        Board of Directors and (v) to the "Administrator" shall mean the Cisco
        Board of Directors or the Cisco Compensation Committee, as appropriate.

                (b) The award date and the expiration date of each assumed
        Calista Option and all other provisions which govern either the exercise
        or the termination of the assumed Calista Option shall remain the same
        as set forth in the Option Agreement applicable to that option, and the
        provisions of the Option Agreement shall accordingly govern and control
        Optionee's rights under this Agreement to purchase Cisco Stock.

                (c) Your Calista Option shall be assumed by Cisco as of the
        Effective Time. Pursuant to the terms of Amendment No. 1 to your Notice
        of Stock Option Award, which is incorporated by reference in your Option
        Agreement, your option assumed by Cisco in connection with the
        transaction will vest and become exercisable on an accelerated basis
        upon the consummation of the Merger. Accordingly, 25% of the Shares
        subject to your Calista Option vested immediately upon Consummation of
        the Merger, an additional 18.75% of the


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        Shares subject to your Calista Option shall vest one year from the date
        of the consummation of the Merger and an additional 1.5625% of the
        Shares subject to your Calista Option shall vest monthly thereafter.

                (d) For purposes of applying any and all provisions of the
        Option Agreement and/or the Plan relating to Optionee's status as an
        employee or a consultant of Calista, Optionee shall be deemed to
        continue in such status as an employee or a consultant for so long as
        Optionee renders services as an employee or a consultant to Cisco or any
        present or future Cisco subsidiary. Accordingly, the provisions of the
        Option Agreement governing the termination of the assumed Calista
        Options upon Optionee's cessation of service as an employee or a
        consultant of Calista shall hereafter be applied on the basis of
        Optionee's cessation of employee or consultant status with Cisco and its
        subsidiaries, and each assumed Calista Option shall accordingly
        terminate thirty (30)-days following such cessation of service as an
        employee or a consultant of Cisco and its subsidiaries.

                (e) The adjusted exercise price payable for the Cisco Stock
        subject to each assumed Calista Option shall be payable in any of the
        forms authorized under the Option Agreement applicable to that option.
        For purposes of determining the holding period of any shares of Cisco
        Stock delivered in payment of such adjusted exercise price, the period
        for which such shares were held as Calista Stock prior to the Merger
        shall be taken into account.

                (f) In order to exercise each assumed Calista Option, Optionee
        must deliver to Cisco a written notice of exercise in which the number
        of shares of Cisco Stock to be purchased thereunder must be indicated.
        The exercise must be for at least five percent (5%) of the total number
        of Shares subject to the assumed Calista Option (or, if less than five
        percent (5%), the total remaining number of Shares subject to the
        Calista Option). The exercise notice must be accompanied by payment of
        the adjusted exercise price payable for the purchased shares of Cisco
        Stock and should be delivered to Cisco at the following address:

                             Cisco Systems, Inc.
                             170 West Tasman Drive
                             MS 11-3
                             San Jose, CA 95134-1706
                             Attention:  Stock Administration

        4. Except to the extent specifically modified by this Option Assumption
Agreement, all of the terms and conditions of each Option Agreement as in effect
immediately prior to the Merger shall continue in full force and effect and
shall not in any way be amended, revised or otherwise affected by this Stock
Option Assumption Agreement.


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        IN WITNESS WHEREOF, Cisco Systems, Inc. has caused this Stock Option
Assumption Agreement to be executed on its behalf by its duly-authorized officer
as of the 12th day of November, 1999.


                                            CISCO SYSTEMS, INC.


                                            By: LARRY R. CARTER
                                               ---------------------------------
                                               Larry R. Carter
                                               Corporate Secretary


                                 ACKNOWLEDGMENT

        The undersigned acknowledges receipt of the foregoing Stock Option
Assumption Agreement and understands that all rights and liabilities with
respect to each of his or her Calista Options hereby assumed by Cisco are as set
forth in the Option Agreement, the Plan, as applicable, and such Stock Option
Assumption Agreement.


                                          <<FIRST_NAME>> <<LAST_NAME>>, OPTIONEE


DATED: __________________, ____


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