About 4.5 billion years ago when the solar system formed there were a lot of collisions between the existing bodies. One collision formed the Earth moon. A body about the size of Mars slammed into earth. The giant collision spawned a lot of fragments; they eventually amalgamated to form the Moon. Further pout another collision occurred and formed the pluto-Charon plus two addition small moons system, so the answer to the question is COLLISION of two bodies to end up with 4.