No doubt there is something wrong with his set up. If the sag is still at 4 inches he may even have inadequate sized spring bars or they aren't enough. Its not going to work unless he can get his difference within an inch. I would start exactly where you all are thinking... with the set up and weight distribution system. Its obviously wrong and the crux of the problem.This business with the front riding higher, and the spring bars at the trailer frame, invites the supposition that your hitch head needs to be tilted toward the trailer quite a bit more, so the spring bars can provide a lot more force and do so without closely approaching the trailer frame.
If anything, the thing to watch for with a WDH is to make sure the front doesn't sag more than the rear. Your setup is behaving as though the spring force is wholly inadequate.