Xtra Corp Watch

from SC 13D 1 page <page> Cusip No. 984138107 13d Page 11 of 11 Pages Exhibit (4) Required by Item 7 of Schedule 13d Agreement the Undersigned Persons Hereby Agree That Reports on Schedule 13d, and Any Amendments Thereto, Including Such Reports Made by or as Part of a Tender Offer Statement on Schedule To, May Be Filed in a Single Statement on Behalf of All Such Persons, and Further, Each Such Person Designates Marc D. Hamburg as Its Agent and Attorney-In-Fact for the Purpose of Executing Any and All Such Reports Required to Be Made by It With the Securities and Exchange Commission. Dated: August 7, 2001 Bx Merger Sub Inc. By: /S/ Marc D. Hamburg Name: Marc D. Hamburg Title: President Berkshire Hathaway Inc. By: /S/ Marc D. Hamburg Name: Marc D. Hamburg Title: Vice President and Chief Financial Officer Warren E. Buffett /S/ Warren E. Buffett

from 10-K405 ~5 pages Second Supplemental Indenture

from POS AM ~20 pages Indenture or similar

from POS AM ~10 pages Indenture or similar

from POS AM ~5 pages Indenture or similar

from S-3 ~20 pages Form of Floating Rate Medim Term Note

from S-3 ~10 pages Form of Fixed Rate Medium Term Note

from S-3 >50 pages Form of Subordinated Indenture

from POS AMI >50 pages Form of Subordinated Indenture

from 10-K ~20 pages Form of Floating Rate Series C Medium Term Note

from 10-K ~10 pages Form of Fixed Rate Series C Medium Term Note

from 10-K ~5 pages First Supllemental Indenture

from 10-K ~5 pages Fourth Supplemental Indenture

from 8-K ~20 pages Floating Rate Medium-Term Note

from 8-K ~10 pages Fixed Rate Medium-Term Note

from 8-K >50 pages Indenture or similar

from 10-Q ~10 pages Restated Limited Guaranty

from S-3 >50 pages Form of Subordinated Indenture

from S-3 >50 pages Form of Indenture

from S-3 1 page Certificate of Elimination of Designation