How much weight can a 4 inch concrete road carry?

I'm sure it depends upon how well you prepare the subsurface. A good foot of hardpack, quality dirt with another six inches to a foot of roadbase underneath four inches of properly laid concrete can offer some major load bearing qualities. If I were you, I would research how the Autobahn and similar roads are constructed. They are built very differently from most highways and Interstates in the United States.