I am an early childhood educator, and your son's behavior is normal. Rather than having your son appologize for his behavior, after comforting the other child who was hurt, ask your son what he thinks he can do to make his friend feel better (hugs, asking the other child if he/she is o.k.). Since he probably does not understand what appologizing means, this may help him to understand that he can try to do something to make someone feel better when he hurts them. To reinforce that it is not o.k. to hurt someone else, you can ask him to use his words to tell the other child why he is angry (to just say/or yell I'm mad!). You may also try asking his teachers at pre-school what they do if one child hits another child. That way your son will get consistent behavioral reinforcements/guidance at home and in pre-school.
My son had the same problem when he started 4 yr old kindergarten, and I was very upset about his behavior. But looking back I think it is largely a maturity issue because now (will be 6 in July) he does not behave that way.
One thing you can do is explain socially correct behavior. We tried that approach and it helped to a degree. When you go to a restaruant ask him if he sees anyone hitting anyone else? Then explain because people don't do that. Use every opportunity you can to explain correct behavior. Also tell him what he can do to help himself not to become upset, such as take deep breaths, count to 10, walk away and do something else until he calms down, tell the teacher he needs a time out because he is getting upset, etc. Help him to verbalize his anger instead of acting it out.
When our son was younger we tried to get him to go in his room and hit his pillow to get rid of his anger, and he said 'I like my pillow and don't want to hurt it'. LOL
Dear Karen,
This all sounds like 'normal' behavior problems that need to be managed.
Any overtly aggressive behavior needs to responded to with immediate time out, with the simple message: "No hurting."
A child so young is incapable of a true apology, because the child cannot have perspective on his behavior. As a parent, you need to urge the apology as a way of teaching, but don't expect it can be truly genuine.
It's not uncommon for children so young to have their major problems in unstructured situations. What this tells you is that you need to increase the amount of supervision and monitoring at such times.
Finally, consider increasing his exposure to pre-school. He really isn't involved in the program to a degree that is sufficient for him to be fully integrated into it.