It's not that it was a bomber or a fighter. It was really both. There was nothing limiting an A to only dropping bombs and a B only carrying guns, for example. There are lots of test setups that never worked and lots of myths about what it could carry. For the rockets, it could only carry 4 on the outer wings (like our 110G). The 6-gun in the bomb bay did not work (or rather it worked with disasterous results, destroying part of the plane it launched from) and was never tested in flight.
P.S. U4 wasn't a sub type, just a naming convention describing what loadout was put into it when it was in the factory.