J.G.
I think it's somewhat normal. Is he 4.5? When my daughter and her friends hit 4.5, they all started doing uncharacterististic things. My daughter started name calling, throwing mad tantrums, etc. it was all shocking. But that's the point: dr Ames says this is a super emotional age, and they play with the extremes.
A friend of mine's son hit her LOTS, but he is the type to get violent when frustrated.
It all calmed down around their bday. Meanwhile, zero tolerance, sent to room for anti-social behavior, no TV with name calling in it, etc.