My Son 4Yr Old Son

That's definitely NOT typical 4 year old behavior. Not at all, I'm sorry. I would say that the separation is definitely a factor, and if he's ever witnessed violence in the home that would be a huge factor. Short time outs are not going to be enough to stop the behavior. I would really encourage family counseling and therapy for your son. If you and your estranged husband are having a contentious divorce, then it might be a good idea to start setting some boundaries for friendlier interactions in front of the children as well co-parenting classes.

I have a four year old boy...and no this is not normal behavior. Yes he gets upset, yes he has hit his sister. I don't think pushing down they stairs and punching is normal behavior.

I would say you need to take him to a counselor, as yes this could be because of the separation and he is reacting to it.

It would be helpful if you could provide a specific example of his behavior and how you are reacting to it/handling it.

There could be lots of reasons/solutions..It would be helpful to know the tone of his home environment in general and what sort of reaction/discipline you have used in the past when he acts aggressively.

I would talk to his teachers at school and see if they could recommend some counseling for you and him. This type of behavior is NOT normal. Yes, a four year old may occasionally "lose it" and lash out at another child but an ongoing pattern of aggression at his age is a warning sign that something is wrong.
I'm not sure if it has to do with the separation or the way you've been disciplining him (?) This is why counseling, or parenting classes, is a good idea, because it will help you get to the heart of the problem and give you tools going forward. Good luck!

I'm starting to think it's the age. I don't know any 4 year olds that arent aggressive. My daughter has recently started grabbing me --something she has never done. They say 4 is a big emotion year, when they start to really learn to control their emotions, etc.

I'm stunned by the behavior of all the 4 year olds I know. Sweet girls that pinch if someone bothers them, boys spitting in people's faces. They all seems to lash out if things don't seem fair.

I have a zero tolerance policy for violence in my house. Only sweet girls get sweets, and if she is really bad, no afternoon videos. And of course TO immediately. When she grabbed me roughly last week, she was in her room for a very long time.....antisocial behavior is immediate isolation in my book.

Have you thought about counseling. He may just be reacting to the big change of you and your husband seperating. It is amazing what kids can feel/perceive without being told directly what is going on. I don't think punching a child in the face is normal behavior. I think this is hugely due to what is going on at home. Unforutnately you can't do much about it besides talk to him and hopefully he will snap out of it. Perhaps going to a counselor will help with finding a way to better manage his behavior. Best of luck.