4x4 is allowed, remember the silver Audi A4's not too long ago.

The problem with most 4x4 cars as explained at the time was that they would understeer through most parts of a corner and would need to take a slightly different line. Wheras FWD has initial understeer it can pull itself through the corner and RWD drives the car through from the rear. 4wd also marginally a heavier setup.

When it rained though the Audis typically walked away with it.