Preschool fighting

If you have a son or daughter how do they defend themselves from bully's?

I have a child that goes to preschool and there was a boy that did not like sharing would hit kids and take away toys punch them in the stomach and I removed child from that class.

It's a new class and he mentioned he doesn't want to go back to school there is a boy that says mean words about him and hits him.

He said the teacher didn't see, I told him to defend himself but he said if he punches the kid will hit him and if he hits him they will go back and forth getting hurt and he said that won't work four year old logic is smart.

He said after the kid hits him he cries and that it hurts and its a five year old boy what can I say or do

How do I teach him to stand up for himself