I receive an error from the cactus ant task when I attempt to use the jboss3x to run tests against JBoss 4.0. It basically states that the task won't run against anything greater than version 3. The interesting thing, however, is that I manually edit the Specification-Version entry in the manifest file in the JBoss run.jar file to be 3.x and cactus runs just fine.
It'd be nice if the runtime check for the version were modified to work with JBoss 4. Another option would be to add a new jboss4x.