encyclo - 1:12 am on Aug 23, 2007 (gmt 0)

Acrobat 4 was released in 1999, over eight years ago. Adobe are four generations further down the line now.

There's not a lot you can do other than finding an old release of Acrobat 4 or similar (which can make a PDF which is 4.x-compatible) and re-doing your PDF, but with version 4.x only, your client's customer won't be able to read very many recent PDF files.