My daughter found a four-leaf clover. I'd show you a picture, but it was so shrivelled up by the time I saw it, I couldn't really tell if it were the genuine article or not. Of course, I had to give her the benefit of the doubt.
Faux Leaf Clovers
Some people confuse clover with Pepperwort (Marsilea quadrifolia), which is also called four-leaf clover, but it's an aquatic plant, so you won't find it in your lawn. Another imposter is Oxalis, a houseplant that resembles clover and can come with four leaves.
The Usual Suspect
Real four-leaf clover occurs in patches of the common White clover (Trifolium repens), which usually has three leaflets. Some people consider it a lawn weed while others call it an important pasture legume. Legumes use bacteria in their roots to grab nitrogen so they don't need fertilizer.
Switches
The ancestor of clover seems to have had many leaflets. Researchers are investigating genetic controls of colour and shape in clover which might lead to their use as an ornamental plant. To get a typical three leaflets, a gene stops the formation of more leaflets. You get a four leaf clover when this suppressor doesn't work. The details are still be worked out, but this could explain why you can find clover with even more than four leaflets. The Guinness World Record for the most leaves on a clover is eighteen by a person in Japan.
On the Look Out
Some people claim it's not that hard to find four-leaf clovers, you just have to look. I've been browsing patches of clover, hoping to get used to the normal pattern so an abnormal one will pop out, but I've had no luck so far. The chances of four-leaf clover mutant popping up seems to depend on environmental factors. So maybe it's just the area or time of year. Have you found a four-leaf clover?