JBAS 4.2.0 and later is Java 5 only.
This is a decision made by Red Hat / JBoss management and is outside my control. At some point JBAS has to drop support for Java 1.4 since otherwise it holds everything back and becomes difficult to support due ot the number of combinations. Most people are already using Java 5, and some are asking for Java 6 already.
JBM 1.3.0 codebase is *currently* Java 1.4 only, and the libraries are built with 1.4.
JBM 1.3.0 should run inside JBAS 4.0.x although it's not something we would recommend since it involves overwriting serveral jars in the AS with the newer version (see userguide). We don't test this configuration either, but it should work, and will allow you to use 1.4 clients. If you are also using EJB3 and/or webservices on your JBAS instance overwriting jars may cause problems with that.
Going ahead, JBM will probably become Java 5 only. Again, this is s we can take advantage of new features and not get held back in the past. It also allows us to align better with Red Hat messaging which is Java 5 only, so we'd have to change to Java 5 sooner rather than later anyway.