SERIES 2005-A SUPPLEMENT TO COLLATERAL AGENCY AGREEMENT

EXHIBIT 99.4

EXECUTION COPY

NAVISTAR LEASING COMPANY

SERIES 2005-A SUPPLEMENT TO

COLLATERAL AGENCY AGREEMENT

AMONG

HARCO LEASING COMPANY, INC.,

NAVISTAR LEASING COMPANY,

THE SERIES 2005-A PORTFOLIO INTEREST

OBLIGORS IDENTIFIED HEREIN,

THE SERIES 2005-A SECURED PARTIES

IDENTIFIED HEREIN,

NAVISTAR FINANCIAL CORPORATION,

AS SERVICER,

NAVISTAR FINANCIAL RETAIL RECEIVABLES CORPORATION,

NAVISTAR FINANCIAL 2005-A OWNER TRUST,

THE BANK OF NEW YORK,

AS INDENTURE TRUSTEE,

AND

THE BANK OF NEW YORK TRUST COMPANY, N.A.,

AS COLLATERAL AGENT

DATED AS OF JULY 27, 2005

TABLE OF CONTENTS

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SERIES 2005-A SUPPLEMENT TO

COLLATERAL AGENCY AGREEMENT

SERIES 2005-A SUPPLEMENT TO COLLATERAL AGENCY AGREEMENT (this “ Series 2005-A Collateral Supplement ”), dated and effective as of July 27, 2005, among Harco Leasing Company, Inc., a Delaware corporation (“ Harco Leasing ”), Navistar Leasing Company, a Delaware statutory trust (the “ Titling Trust ”), the Series 2005-A Portfolio Interest Obligors identified herein, the Series 2005-A Secured Parties identified herein, and each other person from time to time joining in this Series 2005-A Collateral Supplement in the capacity of a Secured Party, Navistar Financial Corporation, a Delaware corporation (“ Navistar Financial ”), as Servicer, Navistar Financial Retail Receivables Corporation, a Delaware corporation (“ NFRRC ”), Navistar Financial 2005-A Owner Trust, a Delaware statutory trust (the “ Issuer ”), The Bank of New York Trust Company, N.A., a national banking association, as successor-in-interest to Harris Trust and Savings Bank, acting in its capacity as collateral agent for the holders of the Secured Obligations (in such capacity, the “ Collateral Agent ”), and The Bank of New York, a New York banking corporation, acting in its capacity as Indenture Trustee pursuant to the Indenture (as defined below) (the “ Indenture Trustee ”) .

RECITALS

A. Harco Leasing, the Titling Trust, each other Person from time to time joining in this Series 2005-A Collateral Supplement in the capacity of a Secured Party, Navistar Financial and the Collateral Agent have entered into that certain Collateral Agency Agreement, dated as of April 15, 1999 (as amended, the “ Collateral Agency Agreement ”), which provides, among other things, for the grant of certain security interests over the Titling Trust Assets in accordance with the Titling Trust Agreement.

B. Harco Leasing, the General Interest Trustee and the Delaware Trustee have entered into the Titling Trust Agreement, pursuant to which the Titling Trust has been formed for the purpose of taking assignments and conveyances of, holding in trust and dealing in, various Titling Trust Assets.

C. Harco Leasing, the General Interest Trustee, the Delaware Trustee and the Series 2005-A Portfolio Trustee have entered into the Series 2005-A Portfolio Supplement for the purpose of designating and accounting for as separate the Series 2005-A Portfolio Assets as a separate Portfolio Interest known as the Series 2005-A Portfolio Interest, to be represented by the Series 2005-A Portfolio Certificate.

D. The Titling Trust, Harco Leasing, the Collateral Agent and the Servicer have entered into the Titling Trust Servicing Agreement, which provides for, among other things, the servicing of the Titling Trust Assets by the Servicer.

E. Pursuant to the Collateral Agency Agreement, the Collateral Agent holds the Titling Trust Estate as Collateral Agent in order to secure the payment of the Secured Obligations.

F. The Collateral Agency Agreement contemplates that, from time to time, a Titling Trust Interest Holder may desire to secure Secured Obligations with the Designated Collateral identified in a Collateral Supplement.

G. In connection with the creation of the Series 2005-A Portfolio Interest, Harco Leasing has transferred its interest therein and in the related Series 2005-A Portfolio Certificate and Series 2005-A Portfolio Assets to Navistar Financial pursuant to the Lease Purchase Agreement. Navistar Financial has in turn transferred the Series 2005-A Portfolio Certificate and such other collateral to NFRRC pursuant to the Purchase Agreement. NFRRC has in turn transferred the Series 2005-A Portfolio Certificate and such other collateral to the Issuer pursuant to the Pooling Agreement. The Issuer has in turn granted a security interest in the Series 2005-A Portfolio Certificate and such other collateral to the Indenture Trustee pursuant to the Indenture.

H. The parties hereto wish to supplement the terms of the Collateral Agency Agreement (i) to cause the obligations of Harco Leasing under the Lease Purchase Agreement, the obligations of Navistar Financial under the Purchase Agreement, the obligations of NFRRC under the Pooling Agreement and the obligations of the Issuer under the Indenture (each such Person in such capacity, a “ Series 2005-A Portfolio Interest Obligor ”), in each case, to be Secured Obligations, (ii) to establish that each of Navistar Financial under the Lease Purchase Agreement, NFRRC under the Purchase Agreement, the Issuer under the Pooling Agreement and the Indenture Trustee under the Indenture shall, in such capacity, be a Secured Party under the Collateral Agency Agreement (each such person in such capacity, a “ Series 2005-A Secured Party ”) and (iii) to establish the terms on which the Collateral Agent will act on behalf of such Series 2005-A Secured Parties.

NOW THEREFORE, in consideration of the promises and the mutual covenants contained herein and in the Titling Trust Agreement, the parties hereto agree to the following supplemental obligations and provisions with regard to the Series 2005-A Portfolio Assets, the Series 2005-A Portfolio Interest and the Series 2005-A Portfolio Certificate:

ARTICLE X

DEFINITIONS

SECTION 10.1 Definitions . For all purposes of this Series 2005-A Collateral Supplement, except as otherwise expressly provided or unless the context otherwise requires:

(a) unless otherwise defined herein, all capitalized terms used herein shall have the meanings attributed to them by Appendix A to the Pooling Agreement dated as of July 27, 2005 between Navistar Financial Retail Receivables Corporation and Navistar Financial 2005-A Owner Trust (the “ Pooling Agreement ”), or if not defined therein, then as defined in Appendix A to the Titling Trust Agreement (as defined in the Pooling Agreement);

(b) the rules of construction set forth in Part II of Appendix A to the Pooling Agreement shall be applicable to this Series 2005-A Collateral Supplement; and

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(c) any reference herein to the “ General Interest Trustee, acting on behalf of the Titling Trust ,” or words of similar import, shall be deemed to mean the General Interest Trustee, ac