According to Wikipedia, the four leaf clover is "an uncommon variation of the common, three-leavedclover. According to tradition, such leaves bring goodluck to their finders, especially if found accidentally." It is not known if the little extra is due to environmental factors or a genetic variant, but I like to think they are, as Rose Mordi of Nigeria put it, a "naturally occurring chromosomal arrangement".
The four leaves are believed to represent faith, hope, love, and luck. All Americans like to tout their Irish roots on March 17th but we celebrate our lucky little extra all year long.