<PAGE>

                                                                    EXHIBIT 10.5

                                 AMENDMENT NO. 4

     AMENDMENT NO. 4, dated as of September 24, 1999 (this "Amendment"), to the
Fourth Amended and Restated Credit Agreement, dated as of December 21, 1998, by
and among Insight Communications Company, L.P., the Lenders party thereto, CIBC
Inc. and Fleet Bank, N.A., as Co-Agents, and The Bank of New York, as Agent and
as Issuing Bank, as amended by Waiver No. 1 and Amendment No. 1, dated as of
December 21, 1998, Waiver No. 2 and Amendment No. 2, dated as of December 31,
1998, and Amendment No. 3, dated as of July 1, 1999 (as so amended, the "Credit
Agreement").

                                    RECITALS

     I. Capitalized terms used herein and not defined herein shall have the
meanings assigned to such terms in the Credit Agreement.

     II. The Borrower has proposed to engage in a series of transactions as
follows:

          A. The Borrower has agreed to acquire 50.0% of the partnership
     interests in InterMedia Capital Partners VI, L.P.

          B. The Borrower and a subsidiary of AT&T Broadband & Internet Services
     will form a limited partnership and a limited liability company, each to be
     50.0% owned by each of them.

          C. Each of the Borrower and AT&T Broadband & Internet Services will
     cause all of the equity interests in each of Indiana and InterMedia Capital
     Partners VI, L.P. to be contributed to the newly formed limited
     partnership.

     III. In connection with the foregoing, the Borrower has requested that the
Agent agree to amend the Credit Agreement upon the terms and conditions
contained herein, and the Agent is willing so to agree.

     Accordingly, in consideration of the Recitals and the terms and conditions
hereinafter set forth, and for other good and valuable consideration, the
receipt and adequacy of which are acknowledged, the Borrower and the Agent agree
as follows:

     1. Section 1.1 of the Credit Agreement is amended by (a) deleting the
defined terms "Consolidated Annualized Cash Flow", "Knafel", "Negative Pledge
Agreement", and "Willner" therefrom, and (b) deleting the following from the end
thereof:

          Each of the following terms shall have the meaning set forth in
     Amendment No. 3, dated as of July 1, 1999, to the Credit Agreement
     ("Amendment No. 3"):

          "Corporate Restructuring"
          "Insight Holdings"
          "IPO"
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


     2. Section 1.1 of the Credit Agreement is further amended by deleting the
defined term "Super-majority
Lenders" therefrom.

     3. Section 1.1 of the Credit Agreement is further amended by adding the
following defined terms thereto in appropriate alphabetical order:

          "Approved Subordinated Debt": Indebtedness for borrowed money incurred
     by the Borrower to Insight Holdings, provided that (a) the maturity date
     therefor is not earlier than December 31, 2006, (b) the terms and
     conditions thereof are less restrictive upon the Borrower and the
     Subsidiaries than those contained in the Loan Documents, and (c) such
     Indebtedness, together with all interest thereon and fees payable in
     respect thereof, is subordinated to the obligations of the Credit Parties
     under the Loan Documents pursuant to the Approved Subordination Agreement.

          "Approved Subordination Agreement": a subordination agreement,
     executed and delivered by the Borrower and Insight Holdings to the Agent,
     substantially in the form of Exhibit L hereto.

          "Effective Acquisition Cost": with respect to any Acquisition, the
     Acquisition Cost less the sum of each of the following to the extent
     included therein: (a) cash consideration paid or agreed to be paid from the
     proceeds of (i) the incurrence by the Borrower of Approved Subordinated
     Debt, and/or (ii) additional capital contributed to the Borrower from
     Insight Holdings in the form of cash, and (b) consideration in the form of
     common stock of Insight Holdings.

          "IM6": as defined in Section 8.3(f).

          "IM6 Acquisition": as defined in Section 8.3(f).

          "Insight Holdings": Insight Communications Company, Inc., a Delaware
     corporation.

          "Insight-TCI JV Companies": as defined in Section 8.5(i).

          "IPO": the initial public offering of shares of common stock of
     Insight Holdings.

          "Source Media Investment": any and all Investments made pursuant to
     Section 8.5(j).

          "System Cash Flow": as of any date of determination, an amount equal
     to (a) gross revenues minus operating expenses (other than corporate
     overhead and administrative expenses), in each case for the fiscal quarter
     most recently ended in respect of which the Borrower shall have delivered
     the Compliance Certificate pursuant to Section 7.1(c), directly
     attributable to the cable television systems owned and operated by the
     Borrower and the Restricted Subsidiaries,


                                       2
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


     multiplied by (b) four. Notwithstanding anything to the contrary contained
     in this definition, for purposes of determining "System Cash Flow" only,
     all Acquisitions, Dispositions and Exchanges occurring during a fiscal
     quarter shall be deemed to have occurred on the first day of such quarter.

          "Vested Employees": certain employees of the Borrower and/or the
     Restricted Subsidiaries.

     4. Each of the following terms contained in Section 1.1 of the Credit
Agreement is amended and restated in its entirety as follows:

          "Collateral Documents": collectively, the Borrower Security Agreement,
     the Subsidiary Guaranty, the Approved Subordination Agreement, if any, the
     Pledge Agreement, and all documents executed or delivered in connection
     with any of the foregoing.

          "Consolidated Cash Flow": for any period, net income of the Borrower
     and its Restricted Subsidiaries for such period, determined on a
     Consolidated basis in accordance with GAAP plus (i) the sum of, without
     duplication, each of the following with respect to the Borrower and its
     Restricted Subsidiaries, to the extent utilized in determining such net
     income: (a) all interest expense, and (b) depreciation, amortization and
     other non-cash charges, minus (ii) the sum of, without duplication, each of
     the following with respect to the Borrower and its Restricted Subsidiaries,
     to the extent utilized in determining such net income: (a) extraordinary
     gains and losses from sales, exchanges and other dispositions of Property
     not in the ordinary course of business, (b) other non-recurring items, and
     (c) management fees that accrued during such period that were not paid
     during such period or at any time before the date of delivery, pursuant to
     Section 7.1(c), of the Compliance Certificate for the last full fiscal
     quarter included in such period, minus (iii) all Restricted Payments made
     by the Borrower pursuant to Section 8.6(iii) during such period.

          "Control Group": collectively, Sidney Knafel and his Family Group,
     Michael Willner and his Family Group and Kim Kelly and her Family Group.

          "Excluded Transactions": the Americable Acquisition, the Falcon Swap,
     the Source Media Investment and the IM6 Acquisition.

     5. The term "Capital Expenditures" contained in Section 1.1 of the Credit
Agreement is amended by adding the following sentence to the end thereof:

     Notwithstanding anything to the contrary contained in this defined term,
     "Capital Expenditures" shall exclude, to the extent otherwise included
     therein, Capital Expenditures directly attributable to the IM6 Acquisition.

     6. The term "Adjusted Consolidated Annualized Cash Flow" contained in
Section 1.1 of the Credit Agreement is amended by replacing the term
"Consolidated Annualized Cash Flow"


                                       3
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


appearing therein with "Consolidated Cash Flow".

     7. The term "Consolidated Fixed Charges" contained in Section 1.1 of the
Credit Agreement is amended by replacing the reference "Section 8.6(ii)" with
"paragraphs (ii) and (iv) of Section 8.6".

     8. Clause (v) of the term "Excess Cash Flow" contained in Section 1.1 of
the Credit Agreement is amended and restated in its entirety as follows:

     (v) Restricted Payments made pursuant to paragraphs (ii) and (iv) of
     Section 8.6 during such fiscal year,

     9. The term "Special Counsel" contained in Section 1.1 of the Credit
Agreement is amended by deleting the name "Emmet, Marvin & Martin, LLP" and
inserting in its place the name "Bryan Cave, LLP".

     10. The term "Unrestricted Subsidiary" contained in Section 1.1 of the
Credit Agreement is amended by adding the terms "Insight-TCI LP, Insight-TCI
LLC," immediately after the term "ICCO Operating,".

     11. Section 7.1(c) of the Credit Agreement is amended and restated in its
entirety as follows:

          (c) Within 45 days after the end of each of the first three fiscal
     quarters (90 days after the end of the last fiscal quarter) of each fiscal
     year (i) a Compliance Certificate, certified by a Financial Officer of the
     Borrower, and (ii) a copy of a financial statement setting forth the System
     Cash Flow of the Borrower and the Restricted Subsidiaries on a Consolidated
     basis for such quarter, setting forth in each case in comparative form the
     figures for the corresponding quarter of the immediately preceding fiscal
     year, certified by a Financial Officer of the Borrower as presenting fairly
     in all material respects the System Cash Flow of the Borrower and the
     Restricted Subsidiaries on a Consolidated basis for such quarter.

     12. Section 8.1 of the Credit Agreement is amended by (a) replacing the
amount "$5,000,000" in clause (iii) thereof with "$10,000,000", (b) deleting the
word "and" at the end of clause (vi) thereof, and (c) amending and restating
clause (vii) thereof as follows:

     (vii)(a) Indebtedness of the Borrower in an aggregate principal amount not
     to exceed $25,000,000 at any one time outstanding, (b) Approved
     Subordinated Debt of the Borrower, and/or (c) Indebtedness of the Borrower
     pursuant to a guarantee of up to $30,000,000 of Indebtedness of certain
     subsidiaries of IM6 (the "IM6 Guarantee"), provided that, immediately
     before and after giving effect to each incurrence of Indebtedness under
     this clause (vii), no Default shall or would have occurred and be
     continuing and the Borrower shall be in pro-forma compliance with Section
     7.11.

     13. Sections 8.3(f) and 8.3(g) of the Credit Agreement are amended and
restated in their entirety as follows:


                                       4
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


          (f) provided that both immediately before and after giving effect
     thereto, no Default shall or would exist

               (1) the Falcon Swap,

               (2) the Acquisition of certain cable television systems located
          in Indiana and previously identified to the Lenders from Americable
          International - Michigan - Inc. and certain affiliates thereof for
          total consideration not in excess of $10,850,000 (exclusive of normal
          course adjustments) in cash (the "Americable Acquisition"), and/or

               (3) the Acquisition by the Borrower of 50.0% of the partnership
          interests in InterMedia Capital Partners VI, L.P. ("IM6"), together
          with all of the limited partnership interests in each of the
          subsidiaries of IM6 that are not owned by IM6, for $327,185,000
          (subject to adjustment as provided in the purchase agreement therefor)
          in cash (the "Cash Consideration") plus the assumption of
          approximately $430,000,000 million in debt (the "IM6 Acquisition"),
          provided that (1) it is consummated in all material respects in
          accordance with such purchase agreement, and (2) during the period
          beginning on July 1, 1999 and ending on the date of the IM6
          Acquisition, Insight Holdings shall have contributed to the Borrower
          in the form of additional capital, an aggregate amount in cash equal
          to no less than the sum of the Cash Consideration plus an amount equal
          to all fees and expenses payable by the Borrower and the Restricted
          Subsidiaries in connection with the IM6 Acquisition, in each case to
          the extent due and payable on or prior to the closing thereof; and

          (g) other Acquisitions, provided, however, that (1) immediately before
     and after giving effect to each Acquisition made pursuant to this paragraph
     (g) that has an Effective Acquisition Cost in excess of $1.00, the ratio of
     Consolidated Total Debt to System Cash Flow shall be less than or equal to
     5.00:1.00, (2) the Acquisition Cost in respect of all Acquisitions (other
     than Aquisitions of cable television systems) made pursuant to this
     paragraph (g) shall not exceed $10,000,000 in the aggregate during any
     fiscal year or $25,000,000 in the aggregate during the period from the
     Effective Date to the termination of this Agreement, (3) immediately before
     and after giving effect to each Acquisition made pursuant to this paragraph
     (g), no Default shall or would exist, (4) the Borrower will be in
     compliance with each of the financial covenants contained in Section 7.11
     on a pro-forma basis after giving effect to such Acquisition and any
     Indebtedness incurred or assumed in connection therewith, (5) immediately
     after giving effect to each such Acquisition, all of the representations
     and warranties contained in Section 4 shall be true and correct as if then
     made (except to the extent such representations and warranties specifically
     relate to an earlier date, in


                                       5
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


     which case such representations and warranties shall have been true and
     correct on and as of such earlier date), and the Agent shall have received
     a certificate of a Financial Officer of the Borrower to such effect, (6)
     the Agent and the Lenders shall have been given five Business Days' prior
     written notice thereof, (7) the Agent shall have received a certificate
     signed by a Financial Officer of the Borrower, identifying the Person or
     Property to be acquired, the name of the Person making such Acquisition and
     setting forth the total consideration to be paid in respect of such
     Acquisition, (8) the conditions of Section 8.14 shall have been satisfied,
     and (9) the Agent shall have received such other information or documents
     as the Agent shall have reasonably requested.

     14. Section 8.4(c) of the Credit Agreement is amended by (a) replacing the
percentage "10%" in clause (ii) thereof with "30%", and (b) replacing the
percentage "25%" in clause (iii) thereof with "50%".

     15. Section 8.4(d) of the Credit Agreement is amended by replacing the term
"Super-majority" with "Required".

     16. Section 8.5 of the Credit Agreement is amended by (i) replacing the
amount "$8,000,000" in paragraph (g) thereof with "$15,000,000", (ii) deleting
the word "and" at the end of paragraph (g) thereof, (iii) replacing the period
at the end of paragraph (h) thereof with a semi-colon, and (iv) adding the
following to the end thereof:

          (i)(1) the formation by the Borrower and TCI of Indiana Holdings, Inc.
     ("TCI Holdings") of a limited partnership ("Insight-TCI LP") and a limited
     liability company ("Insight-TCI LLC" and, together with Insight-TCI LP, the
     "Insight-TCI JV Companies"), each to be 50.0% owned by the Borrower and TCI
     Holdings, (2) the contribution by the Borrower of $15,000 to Insight-TCI
     LLC, (3) the contribution by the Borrower to Insight-TCI LP of all of the
     partnership interests held by the Borrower in IM6 and the subsidiaries
     thereof (collectively, the "IM6 Contribution"), provided that TCI Holdings
     causes all of the remaining partnership interests in IM6 to be contributed
     to Insight-TCI LP substantially contemporaneously therewith, and/or (4) the
     contribution by the Borrower to Insight-TCI LP of all of the membership
     interests held by the Borrower in Indiana, provided that (A) TCI Holdings
     causes all of the remaining membership interests in Indiana to be
     contributed to Insight-TCI LP substantially contemporaneously therewith,
     and (B) it is made contemporaneously with the IM6 Contribution; and

          (j) the Borrower may (1) purchase common stock and common stock
     purchase warrants in Source Media, Inc. for cash consideration not in
     excess of $12,000,000, and/or (2) purchase a 50.0% interest in a joint
     venture to be formed by the Borrower and Source Media, Inc. for cash
     consideration not in excess of $13,000,000, provided that with respect to
     each Investment pursuant to this paragraph (j), no Default shall exist
     immediately before or after giving effect to such Investment.


                                       6
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


     17. Section 8.6 of the Credit Agreement is amended by (a) deleting the word
"and" at the end of paragraph (i) thereof, (b) deleting the period at the end of
paragraph (ii) thereof, and (c) adding the following to the end thereof:

          (iii) the Borrower may at any time and from time to time declare and
     pay dividends and other distributions to Insight Holdings in an amount
     equal to the Borrower's pro rata share of all out-of-pocket administrative,
     legal, accounting, and stock transfer expenses incurred by Insight
     Holdings, provided that, immediately before and after giving effect
     thereto, no Default shall or would have occurred; and

          (iv) the Borrower may at any time and from time to time declare and
     pay dividends and other distributions to Insight Holdings in an amount
     equal to the Borrower's pro rata share of all out-of-pocket advisory fees
     incurred by Insight Holdings to Non-affiliates, provided that, immediately
     before and after giving effect thereto, no Default shall or would have
     occurred. For purposes hereof, "Non-affiliates" means any Person other than
     (a) any Affiliate or other affiliate of Insight Holdings or the General
     Partner, including any subsidiary of Insight Holdings or the General
     Partner, (b) any partner, director, officer or employee of any Person
     listed in clause (a) hereof, or (c) any member of a Family Group of any
     Person listed in clauses (a) or (b) hereof.

     For purposes of the preceding clauses (iii) and (iv) of this Section 8.6,
     the Borrower's "pro rata share" shall be (a) 100% until such time, if any,
     as Insight Holdings has any direct investment in any Person or Operating
     Entity (other than the Borrower), and (b) thereafter, such percentage as
     shall reasonably approximate the proportion that the Borrower's gross
     revenues bears to the sum of (x) the Borrower's gross revenues, plus (y)
     the gross revenues attributable to all such direct investments.

     18. Section 8.7 of the Credit Agreement is amended by replacing the amount
"$15,000,000" appearing therein with "$20,000,000".

     19. Section 8.14(c) of the Credit Agreement is amended by (a) inserting the
designation "(X)" immediately after the phrase "the Borrower or any of its
Restricted Subsidiaries may create or acquire", and (b) inserting the phrase ",
and/or (Y) the Insight-TCI JV Companies" immediately before the period at the
end thereof.

     20. Each of Sections 9.1(f) and 9.1(j) of the Credit Agreement is amended
by inserting the term ", Insight Holdings" immediately after the term
"Restricted Subsidiaries" in each place it appears therein.

     21. Section 9.1(h) of the Credit Agreement is amended by inserting the term
", Insight Holdings" immediately after the phrase "the General Partner," in each
place it appears therein.

     22. Section 9.1(i) of the Credit Agreement is amended by inserting the term
", Insight


                                       7
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


Holdings" immediately after the phrase "the General Partner" in each place it
appears therein.

     23. Section 9.1(n) of the Credit Agreement is amended and restated in its
entirety as follows:

          (n) (1) the Borrower shall cease to be, directly or indirectly, a
     wholly-owned Subsidiary of Insight Holdings, (2) any of the Capital Stock
     of the Borrower shall be subject to any Lien (other than Liens (i) securing
     the obligations of the Borrower and the Subsidiaries under the Loan
     Documents, and/or (ii) arising by operation of law in the ordinary course
     of business of Insight Holdings), (3) the Control Group shall fail to
     beneficially own shares of the issued and outstanding common stock of
     Insight Holdings representing at least 25.0% of the number of votes of all
     classes of common stock of Insight Holdings, voting as a single class, or
     (4) any Person or "group" (within the meaning of the Securities Exchange
     Act of 1934 and the rules of the Securities and Exchange Commission
     thereunder as in effect on the date hereof), shall hold a greater
     percentage of the number of votes of all classes of common stock of Insight
     Holdings, voting as a single class, than the Control Group; or

     24. All of the obligations, if any, of the Borrower, Sidney R. Knafel, ICC
Associates, L.P., Insight Communications, Inc., and Insight Finance Corporation
under the Negative Pledge Agreement and the Affiliate Subordination Agreement
are hereby terminated, other than the obligations, if any, that by the terms of
such agreements survive termination.

     25. The Credit Agreement is amended by adding a new Exhibit L thereto in
the form of Exhibit L to this Amendment.

     26. Except as otherwise provided in paragraph 27, paragraphs 1 through 25
hereof shall not be effective until such time as each of the following shall
have occurred:

          (a) Required Lenders and each of the Guarantors shall have consented
     hereto in writing, and

          (b) the Borrower shall have paid to the Agent all fees that the
     Borrower may have agreed in writing to pay to the Agent (for its own
     account or for the account of the Lenders) in connection herewith.

     27. Notwithstanding anything to the contrary contained in paragraph 26,
paragraphs 2 and 15 hereof shall not be effective until such time as
Super-majority Lenders shall have consented hereto in writing.

     28. The Borrower (a) reaffirms and admits the validity and enforceability
of each Loan Document and all of its obligations thereunder, (b) agrees and
admits that it has no defense to or offset against any such obligation, and (c)
represents and warrants that, as of the date of the execution and delivery
hereof by the Borrower and assuming the effectiveness of all of the provisions
of this Amendment, no Default has occurred and is continuing, and that each of
the representations and warranties made by it in the Credit Agreement is true
and correct with the


                                       8
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


same effect as though such representation and warranty had been made on such
date, except to the extent such representations and warranties specifically
relate to an earlier date, in which case such representations and warranties
shall have been true and correct on and as of such earlier date.

     29. In all other respects, the Loan Documents shall remain in full force
and effect, and no amendment in respect of any term or condition of any Loan
Document shall be deemed to be an amendment in respect of any other term or
condition contained in any Loan Document.

     30. This Amendment may be executed in any number of counterparts all of
which, when taken together, shall constitute one agreement. In making proof of
this Amendment, it shall only be necessary to produce the counterpart executed
and delivered by the party to be charged.

     31. THIS AMENDMENT IS BEING EXECUTED AND DELIVERED IN, AND IS INTENDED TO
BE PERFORMED IN, THE STATE OF NEW YORK AND SHALL BE CONSTRUED AND ENFORCEABLE IN
ACCORDANCE WITH, AND BE GOVERNED BY, THE INTERNAL LAWS OF THE STATE OF NEW YORK,
WITHOUT REGARD TO PRINCIPLES OF CONFLICT OF LAWS.


                                       9
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


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


                                   INSIGHT COMMUNICATIONS COMPANY, L.P.

                                   By:  ICC ASSOCIATES, L.P.,
                                        the sole general partner thereof

                                   By:  INSIGHT COMMUNICATIONS, INC.,
                                        the sole general partner thereof


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   THE BANK OF NEW YORK,
                                   individually, as Issuing Bank and as Agent


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   CIBC INC.,
                                   individually and as Co-Agent


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: Executive Director,
                                          CIBC Oppenheimer Corp.,
                                          As Agent
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   FLEET BANK, N.A.,
                                   individually and as Co-Agent


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   BANK OF MONTREAL


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   BANKBOSTON, N.A.


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   PNC BANK, NATIONAL ASSOCIATION


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   BANKERS TRUST COMPANY


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________
<PAGE>

             Insight Communications Company, L.P. -- Amendment No. 4


                                   acknowledged and consented to:

                                   INSIGHT FINANCE CORPORATION


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________



                                   INSIGHT HOLDINGS OF OHIO, LLC


                                   By: ________________________________________

                                   Name: ______________________________________

                                   Title: _____________________________________