<PAGE>
                                                                     Exhibit 4.9

                                FOURTH AMENDMENT
             TO THIRD AMENDED AND RESTATED SECURED CREDIT AGREEMENT


         THIS FOURTH AMENDMENT TO THIRD AMENDED AND RESTATED SECURED CREDIT
AGREEMENT (this "AMENDMENT") is entered into as of October 27, 2000, among
QUANTA SERVICES, INC., a Delaware corporation ("BORROWER"), the Lenders, as
defined below, and BANK OF AMERICA, N.A., f/k/a NationsBank, N.A., as
administrative agent for the Lenders (in such capacity, the "AGENT").
Capitalized terms used but not defined in this Amendment have the meaning given
such terms in the Credit Agreement (defined below).

                                    RECITALS

         A. The Borrower is party to that certain Third Amended and Restated
Secured Credit Agreement dated as of June 14, 1999 (as amended by the First
Amendment dated as of September 21, 1999, the Second Amendment dated as of March
21, 2000, the Third Amendment and Consent dated as of June 15, 2000, and as may
be amended, restated or supplemented from time to time, the "CREDIT AGREEMENT"),
among the Borrower, Agent, and the lenders from time to time parties thereto
(each a "LENDER" collectively, "LENDERS").

         B. The Borrower has requested that the "LC Commitment Amount" as
defined in Section 1.1 of the Credit Agreement be increased from $15,000,000 to
$50,000,000.

         C. The Borrower and the Lenders have agreed to amend the Credit
Agreement to amend and restate the definition of "LC Commitment Amount" to
accommodate such increase, subject to the terms and conditions set out in this
Amendment.

         NOW THEREFORE, in consideration of the premises and other good and
valuable consideration, the receipt and sufficiency of which are acknowledged,
the undersigned agree as follows:

         1. Amendment. The definition of "LC Commitment Amount" contained in
Section 1.1 of the Credit Agreement is amended and restated in its entirety as
follows (with the underlined portions showing new or revised language):

            "L/C COMMITMENT AMOUNT" means $50,000,000, as such amount may be
            reduced from time to time pursuant to the terms of this Agreement.

         2. Conditions. This Amendment shall not be effective until each of the
following have been delivered to the Agent or the following conditions have been
otherwise satisfied in the Agent's sole discretion:

            (a) this Amendment signed by the Borrower, the Guarantors, and the
         Majority Lenders;

            (b) the Borrower has delivered to the Agent for the benefit of the
         Lenders a certificate from the Borrower that no Default or Potential
         Default exists under the Credit Agreement; and

            (c) such other documents as the Agent may reasonably request.

<PAGE>
         3. Fees and Expenses. The Borrower agrees to pay the reasonable fees
and expenses of counsel to Agent for services rendered in connection with the
preparation, negotiation and execution of this Amendment.

         4. Representations and Warranties. The Borrower and Guarantors
represent and warrant to the Lenders that they possess all requisite power and
authority to execute, deliver and comply with the terms of this Amendment, which
has been duly authorized and approved by all requisite corporate action on the
part of the Borrower and Guarantors, for which no consent of any Person is
required, and which will not violate their respective organizational documents,
and agree to furnish the Lenders with evidence of such authorization and
approval upon request. The Borrower and Guarantors further represent and warrant
to the Lenders that (a) the representations and warranties in each Credit
Document to which they are a party are true and correct in all material respects
on and as of the date of this Amendment as though made on the date of this
Amendment (except to the extent that (i) such representations and warranties
speak to a specific date or (ii) the facts on which such representations and
warranties are based have been changed by transactions contemplated by the
Credit Agreement), (b) it is in full compliance with all covenants and
agreements contained in each Credit Document to which it is a party, and (c) no
Default or Event of Default has occurred and is continuing.

         5. Scope of Amendment; Reaffirmation; Release. Except as affected by
this Amendment, the Credit Documents are unchanged and continue in full force
and effect. However, in the event of any inconsistency between the terms of the
Credit Agreement as hereby amended and any other Credit Document, the terms of
the Credit Agreement shall control and such other document shall be deemed to be
amended hereby to conform to the terms of the Credit Agreement. All references
to the Credit Agreement shall refer to the Credit Agreement as amended by this
Amendment. The Borrower and Guarantors hereby reaffirm their respective
obligations under, and agree that, all Credit Documents to which they are a
party remain in full force and effect and continue to evidence their respective
legal, valid and binding obligations enforceable in accordance with their terms
(as the same are affected by this Amendment). The Borrower and Guarantors hereby
release the Lenders from any liability for actions or failures to act in
connection with the Credit Documents prior to the date hereof. This Amendment
shall be binding upon and inure to the benefit of each of the undersigned and
their respective successors and permitted assigns.

         6. Miscellaneous.

            (a) No Waiver of Defaults. This Amendment does not constitute a
         waiver of, or a consent to, any present or future violation of or
         default under, any provision of the Credit Documents, or a waiver of
         the Lenders' right to insist upon future compliance with each term,
         covenant, condition and provision of the Credit Documents, and the
         Credit Documents shall continue to be binding upon, and inure to the
         benefit of, the Borrower, Guarantors, and the Lenders and their
         respective successors and assigns.

            (b) Form. Each agreement, document, instrument or other writing to
         be furnished Agent under any provision of this instrument must be in
         form and substance satisfactory to Agent and its counsel.

            (c) Multiple Counterparts.  This Amendment may be executed in any
         number of counterparts with the same effect as if all signatories
         have signed the same document. All counterparts must be construed
         together to constitute one and the same instrument.





                                        2
<PAGE>



            (d) Governing Law.  This Amendment and the other Credit Documents
         must be construed-and their performance enforced-under Texas law.

         7. Entirety. THE CREDIT DOCUMENTS REPRESENT THE FINAL AGREEMENT BETWEEN
THE BORROWER, GUARANTORS AND THE LENDERS AND MAY NOT BE CONTRADICTED BY EVIDENCE
OF PRIOR, CONTEMPORANEOUS, OR SUBSEQUENT ORAL AGREEMENTS BY THE PARTIES. THERE
ARE NO UNWRITTEN ORAL AGREEMENTS AMONG THE PARTIES.


 [SIGNATURES AND GUARANTORS' CONSENT AND AGREEMENT APPEAR ON FOLLOWING PAGES.]




                                        3
<PAGE>
         The Amendment is executed as of the date set out in the preamble to
this Amendment.


                                  QUANTA SERVICES, INC.


                                  By: /s/ James H. Haddox
                                     -------------------------------------------
                                           James H. Haddox
                                           Chief Financial Officer


                                  BANK OF AMERICA, N.A., as Administrative Agent


                                  By: /s/ David A. Johanson
                                     -------------------------------------------
                                           David A. Johanson, Vice President


                                  BANK OF AMERICA, N.A., as a Lender

                                  By: /s/ Craig S. Wall
                                     -------------------------------------------
                                           Craig S. Wall
                                           Senior Vice President

                                  BANK ONE, TEXAS, NATIONAL ASSOCIATION, as
                                  a Documentation Agent and as a Lender


                                  By: /s/ Greg Smothers
                                     -------------------------------------------
                                           Greg Smothers
                                           Vice President


                                  FLEET NATIONAL BANK (F/K/A BANK BOSTON, N.A.),
                                  as a Documentation Agent and as a Lender


                                  By: /s/ Michael M. Parker
                                     -------------------------------------------
                                           Michael M. Parker
                                           Managing Director


                                  CREDIT LYONNAIS NEW YORK BRANCH, as a
                                  Managing Agent and as a Lender


                                  By: /s/ Attila Koc
                                     -------------------------------------------
                                           Attila Koc
                                           Senior Vice President


<PAGE>



                                  THE BANK OF NOVA SCOTIA, as a Managing Agent
                                  and as a Lender


                                  By: /s/ F C H Ashby
                                     -------------------------------------------
                                           F C H Ashby
                                           Senior Manager Loan Operations


                                  NATIONAL CITY BANK, as a Lender


                                  By: /s/ Michael J. Durbin
                                     -------------------------------------------
                                           Michael J. Durbin
                                           Vice President


                                  LASALLE BANK NATIONAL ASSOCIATION, as a
                                  Lender


                                  By: /s/ Richard J. Kress
                                     -------------------------------------------
                                           Richard J. Kress
                                           Vice President


                                  FIRST UNION NATIONAL BANK, as a Lender


                                  By: /s/ Mark B. Felkler
                                     -------------------------------------------
                                           Mark B. Felkler
                                           Senior Vice President


                                  COMERICA BANK, as a Lender


                                  By: /s/ Mark B. Grover
                                     -------------------------------------------
                                           Mark B. Grover
                                           Vice President


                                  THE BANK OF TOKYO-MITSUBISHI, LTD., as a
                                  Lender

                                  By: /s/ J. Fort
                                     -------------------------------------------
                                           J. Fort
                                           Vice President





<PAGE>



                                  CHASE BANK OF TEXAS, N.A., as a Lender


                                  By: /s/ James. R. Dolphin
                                     -------------------------------------------
                                           James R. Dolphin
                                           Senior Vice President


                                  GUARANTY FEDERAL BANK, F.S.B., as a Lender


                                  By: /s/ Kevin J. Hanigan
                                     -------------------------------------------
                                           Kevin J. Hanigan
                                           Senior Vice President


                                  SUNTRUST BANK, ATLANTA, as a Lender


                                  By: /s/ Frank A. Coe
                                     -------------------------------------------
                                           Frank A. Coe
                                           Vice President


                                  By:
                                     -------------------------------------------
                                     Name:
                                          --------------------------------------
                                     Title:
                                           -------------------------------------


                                  BANKERS TRUST COMPANY, as a Lender


                                  By: /s/ Diane F. Rolfe
                                     -------------------------------------------
                                           Diane F. Rolfe
                                           Vice President

<PAGE>
                        GUARANTORS' CONSENT AND AGREEMENT

         As an inducement to the Lenders to execute, and in consideration of the
Lenders' execution of this Amendment, each of the undersigned hereby consents to
this Amendment and agrees that the same shall in no way release, diminish,
impair, reduce or otherwise adversely affect the obligations and liabilities of
the undersigned under their respective Guaranties described in the Credit
Agreement executed by the undersigned, or any agreements, documents or
instruments executed by any of the undersigned, all of which obligations and
liabilities are, and shall continue to be, in full force and effect. This
consent and agreement shall be binding upon the undersigned, and their
respective successors and assigns, and shall inure to the benefit of the
Lenders, and their respective successors and assigns.

                             Advanced Communication Technologies, Inc.
                             Arby Construction, Inc.
                             Austin Trencher, Inc.
                             CCLC, Inc.
                             Computapole, Inc.
                             Conti Communications, Inc.
                             Croce Electric Company, Inc.
                             Crown Fiber Communications, Inc.
                             Dillard Smith Construction Company
                             Driftwood Electrical Contractors, Inc.
                             Edwards Pipeline Company,  Inc.
                             Environmental Professional Associates, Limited
                             Fiber Technology, Inc.
                             Five Points Construction Company
                             GEM Engineering Co., Inc.
                             Golden State Utility Co.
                             H.L. Chapman Pipeline Construction, Inc.
                             Haines Construction Company
                             Harker & Harker, Inc.
                             Intermountain Electric, Inc.
                             IRBY Construction Company
                             Line Equipment Sales Co.
                             Logical Link, Inc.
                             Manuel Bros., Inc.
                             Mears Group, Inc.
                             Mears Pipeline Services, Inc.
                             Metro Underground Services, Inc.
                             NetCom Management Group, Inc.
                             Network Communications Services, Inc.
                             North Pacific Construction Co., Inc.
                             North Sky Communications, Inc.
                             Northern Line Layers, Inc.
                             Pac West Construction, Inc.
                             PAR Electrical Contractors, Inc.
                             P.D.G. Electric Company
                             Potelco, Inc.
                             QSI, Inc.
                             Quanta XVII Acquisition Inc.
                             Quanta XLI Acquisition, Inc.
                             Quanta XLII Acquisition, Inc.
                             Quanta XLIII Acquisition, Inc.
                             Quanta XLV Acquisition, Inc.
                             Quanta XLVI Acquisition, Inc.
                             Quanta XLVII Acquisition, Inc.
                             Quanta XLVIII Acquisition, Inc.
                             Quanta L Acquisition, Inc.
                             Quanta LI Acquisition, Inc.


<PAGE>

                            Quanta LII Acquisition, Inc.
                            Quanta LIII Acquisition, Inc.
                            Quanta LIV Acquisition, Inc.
                            Quanta LV Acquisition, Inc.
                            Quanta LVI Acquisition, Inc.
                            Quanta LVII Acquisition, Inc.
                            Quanta LVIII Acquisition, Inc.
                            Quanta LIX Acquisition, Inc.
                            Quanta LX Acquisition, Inc.
                            Quanta LXI Acquisition, Inc.
                            Quanta LXII Acquisition, Inc.
                            Quanta LXIII Acquisition, Inc.
                            Quanta LXIV Acquisition, Inc.
                            Quanta LXV Acquisition, Inc.
                            Quanta LXVI Acquisition, Inc.
                            Quanta LXVII Acquisition, Inc.
                            Quanta LXVIII Acquisition, Inc.
                            Quanta LXIX Acquisition, Inc.
                            Quanta LXX Acquisition, Inc.
                            Quanta LXXI Acquisition, Inc.
                            Quanta LXXII Acquisition, Inc.
                            Quanta LXXIII Acquisition, Inc.
                            Quanta LXXIV Acquisition, Inc.
                            Quanta Delaware, Inc.
                            Quanta Utility Installation Co., Inc,
                            R. A. Waffensmith & Co., Inc.
                            Ranger Directional, Inc.
                            S.K.S. Pipeliners, Inc.
                            Seaward Corporation
                            Southeast Pipeline Construction, Inc.
                            Spalj Construction Company
                            Specialty Drilling Technology, Inc.
                            Sullivan Welding, Inc.
                            Sumter Builders, Inc.
                            SynerTec Inc.
                            Telecom Network Specialists, Inc.
                            The Ryan Company, Inc.
                            Tom Allen Construction Company
                            TRANS TECH Electric, Inc.
                            Trawick Construction Co.
                            TTM, Inc.
                            TVS Systems, Inc.
                            Underground Construction Co., Inc.
                            Utilco, Inc.
                            Utilities Construction Co., Inc.
                            VCI Telecom, Inc.
                            W.C. Communications, Inc.
                            W.H.O.M. Corporation
                            Wade D. Taylor, Inc.
                            World Fiber, Inc.


                            By: /s/ Brad Eastman
                               -------------------------------------------------
                               Brad Eastman, President or Vice President of
                               each Guarantor




<PAGE>


                             Coast To Coast, LLC

                             By:      Environmental Professional Associates,
                                        Limited, Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             By:      Quanta Services, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Lake Norman Pipeline, LLC

                             By:      Edwards Pipeline Company, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Mears/CPG, LLC

                             By:      Mears Group, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Mears Engineering, LLC

                             By:      Mears Group, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Mears/HDD, LLC

                             By:      Mears Group, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Mears Services, LLC

                             By:      Mears Group, Inc., Its Member

                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President

                             Quanta Services Management Partnership, L.P.

                             By:      QSI, Inc., Its General Partner


                                      By:       /s/ Brad Eastman
                                               ---------------------------------
                                               Brad Eastman, Vice President