<PAGE>
                                                                     EXHIBIT 4.5


                                 AMENDMENT NO. 4
                                       TO
                        PREFERRED STOCK RIGHTS AGREEMENT


     This Amendment No. 4 to Preferred Stock Rights Agreement (this "Amendment")
is dated as of March 9, 2004 between Critical Path, Inc. (the "Company"), and
Computershare Trust Company, Inc. (the "Rights Agent"), with reference to the
following:

     A.   The Company and the Rights Agent entered into that certain Preferred
Stock Rights Agreement, dated as of March 19, 2001, as amended by that certain
Amendment No. 1 to Preferred Stock Rights Agreement, dated as of November 6,
2001, that certain Amendment No. 2 to Preferred Stock Rights Agreement, dated as
of November 18, 2003, and that certain Amendment No. 3 to Preferred Stock Rights
Agreement, dated as of January 16, 2004 (the "Agreement").

     B.   The Company desires to amend the Agreement in certain respects in
order to permit certain investors to purchase securities being offered by the
Company without causing such investors to become Acquiring Persons (as defined
in the Agreement) and thereby trigger the occurrence of a Distribution Date (as
defined in the Agreement).

     C.   Under the Agreement, the Company and the Rights Agent may amend the
Agreement, at any time prior to a Distribution Date, which has yet to occur.

     NOW, THEREFORE, pursuant to Section 27 of the Agreement, the Company and
the Rights Agent hereby amend, effective upon the date hereof, the definition of
the term "Acquiring Person" set forth in Section 1(a) of the Agreement to read
in its entirety as follows:

          "(a) "ACQUIRING PERSON" shall mean any Person who or which, together
with all Affiliates and Associates of such Person, shall be the Beneficial Owner
of 15% or more of the Common Shares of the Company then outstanding, but shall
not include the Company, any Subsidiary of the Company or any employee benefit
plan of the Company or of any Subsidiary of the Company, or any entity holding
Common Shares for or pursuant to the terms of any such plan. Notwithstanding the
foregoing, no Person shall be deemed to be an Acquiring Person as the result of
an acquisition of Common Shares or Exchangeable Shares by the Company which, by
reducing the number of shares outstanding, increases the proportionate number of
shares beneficially owned by such Person to 15% or more of the Common Shares of
the Company then outstanding; provided, however, that if a Person shall become
the Beneficial Owner of 15% or more of the Common Shares of the Company then
outstanding by reason of share repurchases by the Company and shall, after such
share repurchases by the Company, become the Beneficial Owner of any additional
Common Shares of the Company (other than pursuant to a dividend or distribution
paid or made by (i) the Company on the outstanding Common Shares in Common
Shares or pursuant to a split or subdivision of the outstanding Common Shares or
(ii) Exchangeco on the outstanding Exchangeable Shares in Exchangeable Shares or
pursuant to a split or subdivision of the outstanding Exchangeable Shares), then
such Person shall be deemed to be an Acquiring Person unless, upon becoming the
Beneficial Owner of such additional Common Shares of the Company, such Person
does not beneficially own 15% or more of the Common Shares of the Company then
outstanding. Notwithstanding the foregoing:


<PAGE>

          (i)  if the Company's Board of Directors determines in good faith that
     a Person who would otherwise be an "Acquiring Person," as defined pursuant
     to the provisions of this Section 1(a), has become such inadvertently
     (including, without limitation, because (x) such Person was unaware that
     such Person Beneficially Owned a percentage of the Common Shares of the
     Company that would otherwise cause such Person to be an "Acquiring Person,"
     as defined pursuant to the provisions of this Section 1(a), or (y) such
     Person was aware of the extent of the Common Shares of the Company such
     Person Beneficially Owned but had no actual knowledge of the consequences
     of such Beneficial Ownership under this Agreement) and without any
     intention of changing or influencing control of the Company, and if such
     Person divested or divests as promptly as practicable a sufficient number
     of Common Shares so that such Person would no longer be an "Acquiring
     Person," as defined pursuant to the provisions of this Section 1(a), then
     such Person shall not be deemed to be or to have become an "Acquiring
     Person" for any purposes of this Agreement;

          (ii) if, as of the date of this Agreement, any Person is the
     Beneficial Owner of 15% or more of the Common Shares of the Company
     outstanding, such Person shall not be or become an "Acquiring Person," as
     defined pursuant to the provisions of this Section 1(a), unless and until
     such time as such Person shall become the Beneficial Owner of additional
     Common Shares of the Company (other than pursuant to a dividend or
     distribution paid or made by (x) the Company on the outstanding Common
     Shares in Common Shares or pursuant to a split or subdivision of the
     outstanding Common Shares or (y) Exchangeco on the outstanding Exchangeable
     Shares in Exchangeable Shares or pursuant to a split or subdivision of the
     outstanding Exchangeable Shares), unless, upon becoming the Beneficial
     Owner of such additional Common Shares of the Company, such Person is not
     then the Beneficial Owner of 15% or more of the Common Shares of the
     Company then outstanding; and

          (iii) the term "Acquiring Person" shall not mean:

               A.   General Atlantic Partners 74, L.P., a Delaware limited
               partnership, GAP Coinvestment Partners II, L.P., a Delaware
               limited partnership, and GapStar, LLC, a Delaware limited
               liability company (referred to collectively with their respective
               Affiliates and Associates as "GAP") so long as GAP does not
               increase (other than an increase resulting solely from a
               reduction of the number of shares of outstanding Common Stock,
               including by reason of repurchases of Common Stock or
               Exchangeable Shares by the Company, or a dividend or distribution
               paid or made by the Company or Exchangeco on, or a split or
               subdivision of, any shares of their respective capital stock) its
               Beneficial Ownership of Common Stock to a percentage of the
               outstanding shares of Common Stock at any time that is greater
               than: (1) the percentage of the outstanding shares of Common
               Stock represented by the

                                       2
<PAGE>

               sum of (x) the shares as to which GAP has Beneficial Ownership as
               of the close of business on November 18, 2003, assuming, for
               purposes of this calculation, all warrants and other securities
               convertible into or exercisable for Common Stock, or any other
               rights to purchase Common Stock, in all cases where held or
               enjoyed by GAP, are immediately convertible or exercisable, plus
               (y) the shares as to which GAP obtains (or is entitled to obtain)
               Beneficial Ownership directly from the Company pursuant to the
               consummation of the transactions described in that certain
               Convertible Note Purchase and Exchange Agreement (the "2003
               Purchase Agreement"), dated November 18, 2003, by and among the
               Company and the "Coinvestors" party thereto, including, without
               limitation, the Issuable Shares and the Rights Shares (each as
               defined in the 2003 Purchase Agreement); or (2) such lesser
               percentage as to which GAP has Beneficial Ownership following any
               transfer of securities by GAP after consummation of the
               transactions described in the 2003 Purchase Agreement (except
               that this clause A shall pertain only until such time as GAP has
               Beneficial Ownership of less than fifteen percent (15%) of the
               outstanding shares of Common Stock); provided, that any shares of
               Common Stock Beneficially Owned by one or more officers or
               directors of the Company where (x) such officers or directors are
               also included within the term "GAP" and (y) such shares of Common
               Stock were distributed directly by the Company to such officers
               or directors as equity-based compensation, shall be excluded from
               the calculation of the Beneficial Ownership of GAP for purposes
               of the definition of "Acquiring Person" if the inclusion of such
               shares of Common Stock in such calculation, by itself, would
               otherwise cause GAP to be an Acquiring Person;

               B.   Cenwell Limited, Campina Enterprises Limited, Great Affluent
               Limited, Lion Cosmos Limited and Dragonfield Limited (referred to
               collectively with their respective Affiliates and Associates as
               "Cenwell") so long as Cenwell does not increase (other than an
               increase resulting solely from a reduction of the number of
               shares of outstanding Common Stock, including by reason of
               repurchases of Common Stock or Exchangeable Shares by the
               Company, or a dividend or distribution paid or made by the
               Company or Exchangeco on, or a split or subdivision of, any
               shares of their respective capital stock) its Beneficial
               Ownership of Common Stock to a percentage of the outstanding
               shares of Common Stock at any time that is greater than: (1) the
               percentage of the outstanding shares of Common Stock represented
               by the sum of (x) the shares as to which Cenwell has Beneficial
               Ownership as of the close of business on November 18, 2003,
               assuming, for purposes of this calculation, all warrants and
               other securities convertible into or exercisable for Common
               Stock, or

                                       3
<PAGE>

               any other rights to purchase Common Stock, in all cases where
               held or enjoyed by Cenwell, are immediately convertible or
               exercisable, plus (y) the shares as to which Cenwell obtains (or
               is entitled to obtain) Beneficial Ownership directly from the
               Company pursuant to the consummation of the transactions
               described in the 2003 Purchase Agreement, including, without
               limitation, the Exchange Shares and the Rights Shares (each as
               defined in the 2003 Purchase Agreement); or (2) such lesser
               percentage as to which Cenwell has Beneficial Ownership following
               any transfer of securities by Cenwell after consummation of the
               transactions described in the 2003 Purchase Agreement (except
               that this clause B. shall pertain only until such time as Cenwell
               has Beneficial Ownership of less than fifteen percent (15%) of
               the outstanding shares of Common Stock);

               C.   Vectis CP Holdings, LLC (referred to collectively with its
               Affiliates and Associates as "Vectis") so long as Vectis does not
               increase (other than an increase resulting solely from a
               reduction of the number of shares of outstanding Common Stock,
               including by reason of repurchases of Common Stock or
               Exchangeable Shares by the Company, or a dividend or distribution
               paid or made by the Company or Exchangeco on, or a split or
               subdivision of, any shares of their respective capital stock) its
               Beneficial Ownership of Common Stock to a percentage of the
               outstanding shares of Common Stock at any time that is greater
               than: (1) the percentage of the outstanding shares of Common
               Stock represented by the sum of (x) the shares as to which Vectis
               has Beneficial Ownership as of the close of business on November
               7, 2001 (if any), plus (y) the shares as to which Vectis obtains
               Beneficial Ownership directly from the Company pursuant to
               the consummation of the transactions described in that certain
               Stock and Warrant Purchase and Exchange Agreement (the "2001
               Purchase Agreement"), dated November 7, 2001, by and among the
               Company and the "Purchasers" named therein, plus (z) the shares
               as to which Vectis obtains Beneficial Ownership directly
               from the Company pursuant to the rights offering for an aggregate
               amount up to $21,000,000 of Series E Preferred Stock of the
               Company pursuant to which the Company will distribute
               transferable rights to certain holders of Common Stock; or (2)
               such lesser percentage as to which Vectis has Beneficial
               Ownership following any transfer of securities by Vectis after
               consummation of the transactions described in the 2001 Purchase
               Agreement (except that this clause C. shall pertain
               only until such time as Vectis has Beneficial Ownership of less
               than fifteen percent (15%) of the outstanding shares of Common
               Stock); provided, that any shares of Common Stock
               Beneficially Owned by one or more officers or directors of the
               Company where (x) such officers or directors are also included
               within the term

                                       4
<PAGE>

               "Vectis" and (y) such shares of Common Stock were distributed
               directly by the Company to such officers or directors as
               equity-based compensation, shall be excluded from the calculation
               of the Beneficial Ownership of Vectis for purposes of the
               definition of "Acquiring Person" if the inclusion of such shares
               of Common Stock in such calculation, by itself, would otherwise
               cause Vectis to be an Acquiring Person;

               D.   Permal U.S. Opportunities Limited, Zaxis Equity Neutral,
               L.P., Zaxis Institutional Partners, L.P., Zaxis Offshore Limited
               and Zaxis Partners, L.P., (referred to collectively with their
               Affiliates and Associates as "Apex") so long as Apex does not
               increase (other than an increase resulting solely from a
               reduction of the number of shares of outstanding Common Stock,
               including by reason of repurchases of Common Stock or
               Exchangeable Shares by the Company, or a dividend or distribution
               paid or made by the Company or Exchangeco on, or a split or
               subdivision of, any shares of their respective capital stock) its
               Beneficial Ownership of Common Stock to a percentage of the
               outstanding shares of Common Stock at any time that is greater
               than: (1) the percentage of the outstanding shares of Common
               Stock represented by the sum of (x) the shares as to which Apex
               has Beneficial Ownership as of the close of business on January
               16, 2004, assuming, for purposes of this calculation, all
               warrants and other securities convertible into or exercisable for
               Common Stock, or any other rights to purchase Common Stock, in
               all cases where held or enjoyed by Apex, are immediately
               convertible or exercisable, plus (y) the shares as to which Apex
               obtains (or is entitled to obtain) Beneficial Ownership directly
               from the Company pursuant to the consummation of the transactions
               described in the Convertible Note Purchase Agreement (the
               "January Purchase Agreement"), dated January 16, 2004, by and
               among the Company and the "Lenders" party thereto, including,
               without limitation, the Issuable Shares or the Common Shares
               (each as defined in the January Purchase Agreement), plus (z) the
               shares as to which Apex obtains (or is entitled to obtain)
               Beneficial Ownership directly from the Company pursuant to the
               consummation of the transactions described in the Convertible
               Note Purchase Agreement (the "March Purchase Agreement"), dated
               March [__], 2004, by and among the Company and the "Lenders"
               party thereto, including, without limitation, the Issuable Shares
               or the Common Shares (each as defined in the March Purchase
               Agreement); or (2) such lesser percentage as to which Apex has
               Beneficial Ownership following any transfer of securities by Apex
               after consummation of the transactions described in the February
               Purchase Agreement (except that this clause D. shall pertain only
               until such time as

                                       5
<PAGE>

               Apex has Beneficial Ownership of less than fifteen percent (15%)
               of the outstanding shares of Common Stock)."

     The undersigned officer of the Company, being an appropriate officer of the
Company and authorized to do so by resolution of the board of directors of the
Company, hereby certifies to the Rights Agent that this Amendment is in
compliance with the terms of Section 27 of the Agreement.

     This Amendment may be executed in any number of counterparts, each of which
shall be deemed an original, and all of which together shall constitute one
instrument.

              [the remainder of this page intentionally left blank]



                                       6
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
duly executed as of the day and year first above written.

                            CRITICAL PATH, INC.


                            By: /s/  Michael J. Zukerman
                                ------------------------------------------------
                                Name: Michael J. Zukerman
                                Title: General Counsel and Senior Vice President


                            COMPUTERSHARE TRUST COMPANY, INC.


                            By: /s/  Kellie Gwinn
                                ------------------------------------------------
                                Name: Kellie Gwinn
                                Title: Vice President

                            COMPUTERSHARE TRUST COMPANY, INC.


                            By: /s/ John M. Mahl
                                ------------------------------------------------
                                Name: John M. Wahl
                                Title: Corporate Trust Officer