My advice is, unless you have some specific reason, remove the log4j.jar files from your application libraries.
I've set up a special lib directory into which I put all the jar files that my application needs that jboss has -- and I compile against that. But I don't put any of those jars in the final WEB-INF/lib directory. I depend on the classloader to pull them from Jboss at run-time.
I can assure you that I've had dozens of odd problems that were fixed by slimming my WEB-INF/lib directory.