I wouldn't feel comfortable parking a Deuce on 4" of concrete. The last garage I built I had 6" of concrete over 6" of coarse crushed stone. Of course that increased the price of my slab by 50%, but I wanted a heavy duty floor which wasn't going to crack on me.
As pointed out above, the substrate is as important as the slab itself. Use stone as a substrate, not sand or gravel, especially if you live in an area with freezing weather. Stone will drain out any ground water, sand or gravel will absorb water. When the freezing weather hits, any water in your substrate will freeze and try to heave the concrete, causing stress which may lead to cracks especially if you are driving heavy equipment over it.
It is also necessary to cure concrete properly before you put any load on it. Concrete absorbs water to cure, so you have to keep it wet until it is fully cured. It takes concrete 28 days to reach its maximum strength! When I poured my slab, I put a layer of heavy plastic sheeting over the substrate so the water in the concrete would not drain off. It takes several hours extra for the concrete to set up this way, but is well worth it. After it set up I power troweled it. The next day after the surface water had evaporated but the concrete was still damp I applied two coats of concrete curing compound over the concrete to seal the water in.
After the concrete finally cured, it was a dark black color, not the usual gray color. I poured that slab 30 years ago in an unheated garage, and it still has NOT cracked!
