Are four-leaf clovers mutations, developmental errors, or something more complicated?
A recessive gene for four leaves has been identified (Tashiro et al., 2010). The clover is a tetraploid species and together with the fact that environmental conditions may also suppress the development of the fourth leaf, it indeed takes a lot of luck to find one. If it will bring you any is another question though.

In fact they are aiming at making a four-leaved cultivar (source: New Scientist).

Reference
- Tashiro et al., Crop Sci (2010); 50: 1260–8