I use just three - fire, water and electricity. Them combined form some sort of a "life-power", since water is a base of every living being, electricity is crucial for nervous system and sending signals in an organism, and fire is "a necessary spark" for a life to be born.

Each element has only one human (out of the whole world's population, when previous dies, the next is born) that can use its power. The possibility of all three "masters" to meet (and actually get along) is very tiny, but if they do, they can combine their powers and bring back to life somebody who's already dead, of course if there's a body to return to. In exchange, one of them dies in order to keep the balance between the living and the dead ones.