4-leaf clovers are a mutation of the usually 3-leafed White Clover plant, Trifolium repens. One clover is actually one leaf of a larger plant, with 3 leaflets. Mutations can occur due to a low frequency recessive gene or environmental causes. Often the reason for mutation is differentiable from one clover to another. The mutation does not stop at the 4-leafed variety: 5-leafed clovers are not uncommon. However, the more leaflets, the harder they are to find (and the luckier they are): the record is an 18-leaf clover, and the highest I've ever seen is 10-leafed.