This document lists implementation feedback about XML 1.0 implementations known by the XML Core Working Group at this time.
There are XML 1.0 Fourth Edition specific tests--along with other XML tests--available via the XML Test Suite page.
"RXP passes all but one of the tests for errata to XML 1.0 third edition and XML 1.1 first edition. The failure is because it treats all undeclared entities as fatal errors rather than some being merevalidity errors." See Richard Tobin's statement (Member only).