I wouldn't do it. You can find a theme that looks similar. If you are though you will need to use Impex. There is no way to just downgrade or at least I have never heard of a way before.
Even if he could, he really should not even think about it. His issues really are minor concerns and addressed otherwise. Aside from impex not being able to import a vB4 database in vB3, it would break all his forum-, thread-, post or userid. Quite a lot of links pointing to his forum would invariably break or point to completely other things. Impex was never designed as a downgrade tool (or an upgrade tool, for that matter), and it should never be suggested as such lightheartedly.