Please help!

I have four male guppies in a 5 gallon tank. I started with one, added another about three weeks later, then added the last two about a week after that and now I am having a problem with one of the last two that I added. He is nipping all of my other guppies! One of them is missing half of a tail now and I watched him tonight try to nip at his eyes. So I removed the aggressive guppy from the tank.

What do I do now? Is he going to stop? Is this normal? Will my other guppies tail grow back? Please help me!