I have been finding 4, 5, and even 6-leaf clovers for years! It's best to look in areas with grass but are not mowed, the edges of woods I have particular luck with.
Finding a unique clover isn't as lucky as you would think: it's just a matter of finding a huge patch of clovers and convincing yourself there's a 4-leaf one right in front of you. You'll be right a surprising number of times.