The four-leaf clover is a uncommon variation of the common three-leaf clover. Traditionally it is a common activity for children to look for these and it is considered good luck to find one.
People who search for four-leafed clovers note that some patches of ground appear more likely to contain four-leafed examples and that this may be partly due to differing growing conditions, such as soil composition or other environmental factors.
Clovers may have even more than four leaves. According to the Guinness Book of World Records, the highest number of leaves found on a clover has been eighteen. It has been estimated that there are approximately 10,000 three-leaf clovers for every four-leaf clover.
According to legend, each leaf of the clover represents something. The first leaf is for hope, the second leaf is for faith, the third leaf is for love, and the fourth leaf, naturally, is for luck.
Information source: Wikipedia