Today, Anthony Rayner sent me a message, describing the issue he had installing a recent build of Application Express 4.0 on his database. He ran into all sorts of compilation errors. As it turns out, Anthony was using Oracle Database version 10.2.0.1. And that won't work with Application Express 4.0.
Starting with Oracle Application Express 4.0 (most likely due out some time in 2010), the minimum database version will be 10.2.0.3. Database version 10.2.0.2 or less will not work. Application Express 4.0 cannot be installed and function and be supported in the earlier database versions - hacking the installation scripts won't work this time.
I say this with one caveat, though. Application Express 4.0 will still work with XE. Even though the database version of XE is 10.2.0.1, it's not really the same software as 10.2.0.1 of the released database software (i.e., Standard Edition). XE is really 10.2.0.1+.