<PAGE>

                                                                 Exhibit (10)(y)

                     FOURTH AMENDMENT TO CREDIT AGREEMENT
                     ------------------------------------

          THIS FOURTH AMENDMENT TO CREDIT AGREEMENT (this "Agreement") is made
as of this 5th day of June, 2001 between C-COR.NET CORP., a Pennsylvania
corporation ("C-Cor"), BROADBAND CAPITAL CORPORATION, a Delaware corporation
("Broadband," and collectively with C-Cor, the "Borrowers"), THE UNDERSIGNED
BANK PARTIES (individually a "Bank" and collectively, the "Banks") and MELLON
BANK, N.A., a national banking association, as agent for the Banks (the
"Agent").

                             W I T N E S S E T H :


          WHEREAS, the Borrowers, the Banks and the Agent are parties to a
Credit Agreement dated as of August 9, 1999, as amended by that certain First
Amendment to Credit Agreement dated as of December 29, 1999, that certain Second
Amendment to Credit Agreement dated as of November 24, 2000 and that certain
Third Amendment to Credit Agreement dated as of May 17, 2001 (as so amended, the
"Credit Agreement"), pursuant to which the Banks agreed to extend to the
Borrowers a Twenty Million Dollar ($20,000,000.00) revolving credit facility, a
Fifty Million Dollar ($50,000,000.00) standby acquisition facility and a Two
Million Five Hundred Thousand Dollar ($2,500,000.00) term loan (Capitalized
terms used herein but not defined in this Agreement shall have the meanings
ascribed to them in the Credit Agreement.);

          WHEREAS, the Borrowers have repaid the Term Loan in full;

          WHEREAS, the Borrowers have requested that the Credit Agreement be
amended in order to, among other things, amend certain financial covenants; and

          WHEREAS, the Agent and the Banks are willing to grant such request,
subject to the terms and conditions of this Agreement.

          NOW, THEREFORE, in consideration of the mutual promises contained
herein and intending to be legally bound, the Borrowers, the Agent and the Banks
hereby covenant and agree as follows:

          1.  Amendments.  Upon the execution and delivery by the Borrowers and
              ----------
the Banks of this Agreement, the Credit Agreement shall be amended as follows:

               (a)  The following defined term shall be added to Section 1.01,
Certain Definitions, in alphabetical order:

          "Adjusted EBITDA" shall mean EBITDA excluding (i) the restructuring
     expense in the maximum amount of $7,241,000 (after tax adjustments), plus
                                                                          ----
     (ii) non-recurring charges in the maximum amount of $4,100,000
<PAGE>

               (b)  Subsection 2.01(e) shall be deleted in its entirety and
replaced with the following:

          (e)  Maturity. To the extent not due and payable earlier, the
     Revolving Credit Loans shall be due and payable on the Revolving Credit
     Expiration Date. If, for any reason, the Revolving Credit Commitment is not
     extended or renewed beyond the Revolving Credit Expiration Date, all
     amounts owing by the Borrowers hereunder or under the Notes (including,
     without limitation, all Letter of Credit Reimbursement Obligations) shall
     be immediately due and payable and the Agent and/or the Banks shall be
     entitled to exercise the remedies set forth in Article VIII hereof.

               (c)  The pricing grid set forth subsection (a) of Section 2.05,
Interest Rates, shall be deleted in its entirety and replaced with the
following:

<TABLE>
<CAPTION>
--------------------------------------------------------------------------------------------------------------
                                               Applicable Margin
--------------------------------------------------------------------------------------------------------------
     Funded              Prime Rate     LIBOR Rate Options and              Swingline Loan
  Indebtedness            Options        Letter of Credit Fee            Fed Funds Rate Option
   to Adjusted
  EBITDA/EBITDA
      Ratio
--------------------------------------------------------------------------------------------------------------
<S>                       <C>           <C>                              <C>
Less than 1.50             (0.50%)              1.25%                            1.25%
--------------------------------------------------------------------------------------------------------------
Greater than or            (0.25%)              1.50%                            1.50%
equal to 1.50 and
less than 2.25
--------------------------------------------------------------------------------------------------------------
Greater than or            (0.25%)              1.75%                            1.75%
equal to 2.25 and
less than 2.75
--------------------------------------------------------------------------------------------------------------
Greater than or              0.0                 2.0%                            2.0%
equal to 2.75 and
less than 3.75
--------------------------------------------------------------------------------------------------------------
</TABLE>

               (d)  Subsection 3.01(d) shall be deleted in its entirety and
replaced with the following:

          (d)  Letter of Credit Fee.  On the date of issuance (and quarterly
     thereafter), the Borrowers shall pay to the Agent for the account of each
     Bank a fee (the "Letter of Credit Fee") for each Letter of Credit equal to
     (i) the face amount of each issued and outstanding Letter of Credit times
                                                                         -----
     (ii) the Applicable Margin.

                                      -2-
<PAGE>

               (e)  Subsection (a) of Section 7.01, Financial Covenants, shall
be deleted in its entirety and replaced with the following:

          (a)  Funded Indebtedness to Adjusted EBITDA/EBITDA Ratio.  As of the
     last day of the fiscal quarters set forth below, the Funded Indebtedness to
     Adjusted EBITDA Ratio, as measured on a rolling four quarter basis of the
     four most recent quarters, shall not exceed the following ratios:

               Period Ending        Maximum Ratio
               -------------        -------------

               June 30, 2001        2.25:1.00

               September 30, 2001   3.75:1.00

     As of the last day of each fiscal quarter thereafter, the Funded
     Indebtedness to EBITDA Ratio, as measured on a rolling four quarter basis
     of the four most recent quarters, shall not exceed 2.25:1.00.

               (f)  Subsection (c) of Section 7.01, Financial Covenants, shall
be deleted in its entirety and replaced with the following:

          (c)  Current Ratio.  As of the last day of the fiscal quarters ending
     June 30, 2001 and September 30, 2001, the Current Ratio shall not at any
     time be less than 2.50:1.00.  As of the last day of each fiscal quarter
     thereafter, the Current Ratio shall not at any time be less than 1.25:1.00.

          2.  Representations and Warranties.  (a)  The Borrowers hereby
              ------------------------------
represent and warrant to the Agent and the Banks that there is no default, Event
of Default or Potential Default under the Credit Agreement, the Loan Documents,
or any other document executed in connection therewith.

               (b)  Each of the representations and warranties by the Borrowers
in or pursuant to the Credit Agreement or any Loan Document are true and correct
in all material respects on and as of the date of this Agreement as though made
on and as of such date.

          3.  Other Terms Confirmed.  All other terms and conditions of the
              ---------------------
Credit Agreement, including, without limitation, the right of the Agent and the
Banks to CONFESS JUDGMENT, are hereby confirmed and shall remain in full force
and effect without modification. From and after the effectiveness of the
amendments set forth in Section 1 hereof, all references in any document or
instrument to the Credit Agreement shall mean the Credit Agreement as amended by
this Agreement.

          4.  Amendment Fee.  Upon the execution of this Agreement, the
              -------------
Borrowers shall pay to the Agent for the sole account of the Agent an amendment
fee in an amount equal to 0.05% of the Commitments.

                                      -3-
<PAGE>

          5.  No New Indebtedness.  The Borrowers specifically acknowledge and
              -------------------
agree that this Agreement shall not represent in any way the extension of any
additional credit by the Banks to the Borrowers, or the satisfaction of any
indebtedness evidenced by Loan Documents or the Credit Agreement as amended
hereby.

          6.  Counterparts.  This Agreement may be executed in any number of
              ------------
counterparts, each of which shall be deemed an original, but all such
counterparts together shall constitute but one and the same instrument.

          7.  Headings.  The descriptive headings which are used in this
              --------
Agreement are for convenience only and shall not affect the meaning of any
provision of this Agreement.

          8.  Governing Law.  This Agreement shall be governed by and construed
              -------------
under the laws of the Commonwealth of Pennsylvania, without regard to conflict
of laws principles.

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


ATTEST:                                           C-COR.NET CORP.


/s/ Joseph E. Zavacky                             By /s/ W. T. Hanelly
---------------------                                -----------------
By: Joseph E. Zavacky                             Name: W.T. Hanelly
Title: Controller                                  Title: VP, Finance
[CORPORATE SEAL]


ATTEST:                                           BROADBAND CAPITAL CORPORATION


/s/ Joseph E. Zavacky                             By /s/ George M. Savereno
---------------------                                ----------------------
By: Joseph E. Zavacky                             Name: George M. Savereno
Title: Director                                    Title: President
[CORPORATE SEAL]


                                      -4-
<PAGE>

                                    MELLON BANK, N.A., as Issuing Bank and as
                                    Agent for the Banks


                                    By /s/ Joseph N. Butto
                                       -------------------
                                       Name:  Joseph N. Butto
                                       Title: Vice President


                                    MELLON BANK, N.A., individually as a Bank


                                    By /s/ Joseph N. Butto
                                       -------------------
                                       Name:  Joseph N. Butto
                                       Title: Vice President


                                    FIRST UNION NATIONAL BANK, individually as a
                                    Bank


                                    By /s/ Frank Kulp
                                       --------------
                                       Name:  Frank Kulp
                                       Title: Vice President


                                    PNC BANK, NATIONAL ASSOCIATION, individually
                                    as a Bank


                                    By /s/ Thomas J. Fowlston
                                       ----------------------
                                       Name:  Thomas J. Fowlston
                                       Title: Vice President

                                      -5-