The tires are referenced to the rim to be assembled and the height of the profile. Thus 4.00-6 tire is designed to be mounted on a rim of 6 inches of diameter and a height from the rim to the end of tire of 4 inches. Additionally indicates the number of plies making up the tire structure. So "4 Ply" indicates that the tire consists of four canvases. The number of plies is directly related to the weight that is able to support the tire. In sport aviation we use at least 4 plies.