I think there's probably a lot of factors in play here, and you may need to approach from different angles.
Firstly, 4 and half is an age when boys have a testosterone surge. I have two very gentle boys, but both became a bit physical at that age (hence me getting a book out of the library about 4 year old boys
). They calm down again at 5.
Second, school should have tighter control of this. I saw it in my dcs' school - play wrestling actually disguising real fighting that got out of hand. Ofsted commented in an otherwise glowing report.
Third, your DS will find himself in these "games" throughout his school career, so he needs to learn how to step away without feeling that he's losing face.
I would definitely make an appointment with the teacher to discuss, and ask how to work with her.
But I would also read the riot act to DS, tell him that you never want to see that in a report/planner again.
Then talk to him about how to handle these situations, work with him to see where the lines are. When to step back.
So, if he's playing a game and it feels out of control, so he becomes scared, how to get himself to a safe space without fighting back. How to work out which children to avoid in certain settings - DS1 had a child in his class who was very physical and caused no end of incidents with children at primary. DS once came home with a huge bruise the size of a man's hand on his side where he'd been kicked
hard.
Despite that, DS liked him, and thought he was fun. He had to learn to tell the boy that he was happy chatting and playing, but to step back if the boy wanted to play physical games. It worked, DS didn't get hurt again, but still kept a friendship with a very volatile child.