Yea offroad was one of my NES favorites. I wonder why there's support for 5 NES controllers in DPadPro if no gaes used 5 controllers *shrug*
Basically, the support is there because it could be done within the capabilities of the parallel port. NES game support doesn't have much to do with it. Of course, you
could use the NES controllers for TG-16 games... Five players is perfect for Bomberman.
To connect four gamepads to the parallel port, use the wiring diagram for five but leave one of them out.

I haven't successfully used DPadPro. I tried once, like five years ago, couldn't get it to work, then discovered the joys of USB gamepads and haven't looked back. Maybe you'll fare better.