4 wheels vs. 2 wheels

We use either tracks or two wheels + some sort of support because it makes it easier to turn. If you build your robot with 4 wheels, like a car, you have to make the front wheels turn, like a car and add a differential to the rear wheels because in a turn the rear wheels will not rotate with the same speed (see http://en.wikipedia.org/wiki/Differential_%28mechanical_device%29). Using two wheels or tracks, turning is as easy as stopping one motor or running the motors in opposite directions. It is mechanically more simple to use the two wheels and ball roller as opposed to add a differential gear and car-like steering.