4'x6' is pretty small. I would keep it really simple. I also wouldn't use overpasses, as you don't have enough room to make the slope gradual enough that the cars won't get stuck on it, or high enough that the antennas won't cause a problem.
Something like this works great. It's not fancy or complex, but if you're serious about racing with other people, you want to spend as little time as possible recovering from crashes or hitting walls.
