EXHIBIT 4
         to SCHEDULE 13D


                       GAP COINVESTMENT PARTNERS II, L.P.
                                3 Pickwick Plaza
                               Greenwich, CT 06830

                                             January 3, 2001

                                POWER OF ATTORNEY
                                -----------------

                  The undersigned, GAP Coinvestment Partners II, L.P., a
Delaware limited partnership, with its principal office at 3 Pickwick Plaza,
Greenwich, Connecticut, United States of America (the "Partnership"), by its
Managing General Partner, Steven A. Denning, a U.S. citizen, of full legal age,
hereby constitutes and appoints Thomas J. Murphy, a U.S. citizen, of full legal
age, its true and lawful attorney-in-fact and agent, in any and all capacities,
to execute and deliver any and all documents and instruments and to make any
governmental filings on behalf of the Partnership as fully to all intents and
purposes as a General Partner of the Partnership might or could do in person,
hereby ratifying and confirming all that said attorney-in-fact may lawfully do
or cause to be done. This power of attorney shall expire on December 31, 2001.


                                   GAP COINVESTMENT PARTNERS II, L.P.

                                   By:  /s/ Steven A. Denning
                                        ---------------------------------------
                                        Steven A. Denning
                                        Managing General Partner


STATE OF CONNECTICUT    )
                        : ss.
COUNTY OF FAIRFIELD     )


                  On the 3rd day of January 2001, before me personally came
Steven A. Denning, to me known, and known to me to be the individual described
in, and who executed the foregoing document, and he acknowledged to me that he
executed the same.


/s/ Sheila Hughes
------------------------
Notary Public