PXE is built against JDK 5.0 and makes heavy use of its new features and libraries. While there was a previous attempt to provide JDK 1.4 compatibility by using Retroweaver, this turned out to be unfeasible; consequently, this feature has now been removed from the existing ant build. There is a good chance that we will be able to run on 1.4 via Retrotranslator once the new build system based on maven2 is fully working.