JBoss 4.2 use JBossMQ as message provider as default.
With JBoss 4.3 the default JMS provider was switched to JBossMessaging.
We try to run different applications (without cluster) with 4.3 and 5.x and give up because of different problems (e.g. transactions).
I suppose that you will have more problems if you run both version within one cluster because of different (incompatible) client APIs