It all depends on the rider, most people on this sight will say 4-wheelers are more dangerious for this or that, because it is a dirt bike sight. I dirt bikes are easier to wreck for beginners, but if that big heavy @ss four wheeler lands on you, its lights out.
P.S I lump most quad riders in two catagories:
1. quadsum; ussually riding a Yamaha or Suzuki sport quad with blue jeans, work boots, flannel shirt or sleveless t-shirt depending on weather, and mullet, some times with lit smoke. Known for doing donuts in parking area.
2. superhunter quad; has a Polaris 800 bear tracker whatever, winch, front and rear racks, weighs more than my truck, is slower than crap and are a pain in the ass if they ever show up at practice track.
Ya, I hate quads.