What I have seen are motor cycle size plates on the quads and normal size car plates on the....whatever they are, like a Rhino or a Ranger. You know, those 4 wheel big things that you sit in and run it like a car.
I have yet to run one of these guys down and ask him about it. The first one I saw was a quad going the opposite way on the main drag here, right in front of the state police station. I was pulling a camper but I looked out the window as he went by and saw the little plate. The next was a red bigger ORV that had a regular car plate on it. I was walking into the store when I saw it go by.