4 posts are much easier to install. The floor can be thinner, and more importantly, the cars' center of gravity is between the posts at all times. They will not tend to pitch over. The 2 post if loaded poorly with a heavy vehicle, will have a very large twisting force on the 2 floor mounts, trying to pitch the car and lift over. So the criticality of the floor becomes much, much higher for a 2 post.