In response to an enquiry ‘How common are four-leaved clovers and what causes them?’, in BBC Wildlife magazine, October 2013 (vol.31, no.11): 96, Phil Gates, the magazine’s resident botanist, explained:
No one knows how common four-leaved clovers are.
‘If only a few leaves are affected, the cause may be environmental stress, perhaps a temperature shock, temporary drought or even disease during leaf development.
If the whole plant is four-leaved the cause is likely to be genetic, due to a mutation.’

In the British Isles only white clover (Trifolium repens) seems to produce leaves with four leaflets.

Image: Phyzome, Wiki Commons.