EXHIBIT 10.4

AMENDMENT NO. 4 TO THIRD AMENDED AND RESTATED CREDIT AND
SECURITY AGREEMENT

          THIS AMENDMENT NO. 4 TO THIRD AMENDED AND RESTATED CREDIT AND SECURITY
AGREEMENT (this “Amendment”) is entered into as of February 12, 2007, by and
among:

          (1)     QUEST DIAGNOSTICS RECEIVABLES INC., a Delaware corporation
(the “Borrower”),

          (2)     QUEST DIAGNOSTICS INCORPORATED, a Delaware corporation as
initial servicer (together with the Borrower, the “Loan Parties”),

          (3)     VARIABLE FUNDING CAPITAL COMPANY LLC, a Delaware limited
liability company as assignee of Blue Ridge Asset Funding Corporation (“VFCC”),
and WACHOVIA BANK, NATIONAL ASSOCIATION, in its capacity as a Liquidity Bank to
VFCC (together with VFCC, the VFCC Group”),

          (4)     ATLANTIC ASSET SECURITIZATION LLC, a Delaware limited
liability company formerly known as Atlantic Asset Securitization Corp.
(together with its successors, “Atlantic” and together with VFCC, the
“Conduits”), and CALYON NEW YORK BRANCH, in its capacity as a Liquidity Bank to
Atlantic (together with Atlantic, the “Atlantic Group”),

          (5)     WACHOVIA BANK, NATIONAL ASSOCIATION, in its capacity as agent
for the VFCC Group, and CALYON NEW YORK BRANCH, in its capacity as agent for the
Atlantic Group (in such latter capacity, together with its successors in such
latter capacity, the “Atlantic Agent” or a “Co-Agent”), and

          (6)     WACHOVIA BANK, NATIONAL ASSOCIATION, as administrative agent
for the VFCC Group, the Atlantic Group and the Co-Agents (in such capacity,
together with any successors thereto in such capacity, the “Administrative
Agent” and together with each of the Co-Agents, the “Agents”),

with respect to that certain Third Amended and Restated Credit and Security
Agreement dated as of April 20, 2004, by and among the parties hereto (as
heretofore amended, the “Existing Agreement” which, as amended hereby, is
hereinafter referred to as the “Agreement”).

                    Unless otherwise indicated, capitalized terms used in this
Amendment are used with the meanings attributed thereto in the Existing
Agreement.

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W I T N E S S E T H :

                    WHEREAS, the parties hereto desire to extend the Scheduled
Termination Date; and

                    WHEREAS, the parties hereto desire to amend the Existing
Agreement as hereinafter set forth.

                    NOW, THEREFORE, in consideration of the premises and the
mutual agreements herein contained, the parties hereto hereby agree as follows:

                    1.        Amendments to Existing Agreement. Subject to the
terms and conditions hereinafter set forth, the parties hereby agree to amend
the Existing Agreement as follows:

                    1.1      The definition of “Scheduled Termination Date,” is
hereby amended by replacing the phrase “April 20, 2007” with the phrase “July
16, 2007.”

                    2.        Representations.     

                    2.1.     Each of the Loan Parties represents and warrants to
the Lenders and the Agents that it has duly authorized, executed and delivered
this Amendment and that the Agreement constitutes, a legal, valid and binding
obligation of such Loan Party, enforceable in accordance with its terms (except
as enforceability may be limited by applicable bankruptcy, insolvency, or
similar laws affecting the enforcement of creditors’ rights generally or by
equitable principles relating to enforceability).

                    2.2.     Each of the Loan Parties further represents and
warrants to the Lenders and the Agents that each of its representations and
warranties set forth in Section 6.1 of the Agreement is true and correct as of
the date hereof and that no Event of Default or Unmatured Default exists as of
the date hereof and is continuing.

                    3.        Conditions Precedent. This Amendment shall become
effective as of the date first above written upon receipt by the Administrative
Agent of a counterpart hereof duly executed by each of the parties hereto.

                    4.        Miscellaneous.

                    4.1.     Except as expressly amended hereby, the Existing
Agreement shall remain unaltered and in full force and effect, and each of the
parties hereby ratifies and confirms the Agreement and each of the other
Transaction Documents to which it is a party.

                    4.2.     THIS AMENDMENT SHALL BE GOVERNED BY, AND CONSTRUED
IN ACCORDANCE WITH, THE INTERNAL LAWS OF THE STATE OF NEW YORK WITHOUT REFERENCE
TO PRINCIPLES OF CONFLICTS OF LAW.

                    4.3.     EACH LOAN PARTY HEREBY ACKNOWLEDGES AND AGREES
THAT:

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                    4.3.1.     IT IRREVOCABLY (i) SUBMITS TO THE NON-EXCLUSIVE
JURISDICTION, FIRST, OF ANY UNITED STATES FEDERAL COURT, AND SECOND, IF FEDERAL
JURISDICTION IS NOT AVAILABLE, OF ANY NEW YORK STATE COURT, IN EITHER CASE
SITTING IN NEW YORK COUNTY, NEW YORK, IN ANY ACTION OR PROCEEDING ARISING OUT OF
OR RELATING TO THE AGREEMENT, AND (ii) WAIVES, TO THE FULLEST EXTENT IT MAY
EFFECTIVELY DO SO, THE DEFENSE OF AN INCONVENIENT FORUM TO THE MAINTENANCE OF AN
ACTION OR PROCEEDING IN SUCH COURTS.

                    4.3.2.     TO THE EXTENT THAT IT HAS OR HEREAFTER MAY
ACQUIRE ANY IMMUNITY FROM THE JURISDICTION OF ANY COURT OR FROM ANY LEGAL
PROCESS (WHETHER THROUGH SERVICE OR NOTICE, ATTACHMENT PRIOR TO JUDGMENT,
ATTACHMENT IN AID TO EXECUTION, EXECUTION OR OTHERWISE) WITH RESPECT TO ITSELF
OR ITS PROPERTY, IT HEREBY IRREVOCABLY WAIVES SUCH IMMUNITY IN RESPECT OF ITS
OBLIGATIONS UNDER OR IN CONNECTION WITH THE AGREEMENT.

                    4.4.       This Amendment may be executed in any number of
counterparts and by the different parties hereto in separate counterparts, each
of which when so executed shall be deemed to be an original and all of which
when taken together shall constitute one and the same Amendment.

<Signature pages follow>

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                    IN WITNESS WHEREOF, the parties hereto have executed this
Amendment as of the date first above written.

  QUEST DIAGNOSTICS RECEIVABLES INC.         By: /s/       Joseph P. Manory    
Name: Joseph P. Manory     Title:   Vice President and Treasurer              
QUEST DIAGNOSTICS INCORPORATED         By:  /s/       Joseph P. Manory     Name:
Joseph P. Manory     Title:   Vice President and Treasurer              
ATLANTIC ASSET SECURITIZATION LLC         By:  CALYON NEW YORK BRANCH     AS
ATTORNEY-IN-FACT         By:  /s/        Sam Pilcer     Name: Sam Pilcer    
Title:    Managing Director               By:  /s/       Kostantina Kourmpetis  
  Name: Kostantina Kourmpetis     Title:    Managing Director              
CALYON NEW YORK BRANCH, INDIVIDUALLY AND   AS ATLANTIC AGENT         By: 
/s/       Sam Pilcer     Name: Sam Pilcer     Title:    Managing Director      
        By:  /s/       Kostantina Kourmpetis     Name: Kostantina Kourmpetis    
Title:    Managing Director      

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  WACHOVIA BANK, NATIONAL ASSOCIATION,   INDIVIDUALLY, AS ADMINISTRATIVE AGENT
AND   AS VFCC AGENT         By:  /s/       Elizabeth R. Wagner     Name:
Elizabeth R. Wagner     Title:    Managing Director               VARIABLE
FUNDING CAPITAL COMPANY LLC         By: WACHOVIA CAPITAL MARKETS, LLC, ITS  
ATTORNEY-IN-FACT         By:  /s/       Douglas R. Wilson, Sr.     Name: Douglas
R. Wilson, Sr.     Title:    Vice President

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