Op, I would have reacted like you, because the instinct to protect my DC was already on high alert. It was only when my exDP hit our son that I realised that I couldn't trust him and that actually I had felt like that for a long time . There was an underlying fear that I hadn't admitted to myself. It turned out to be a valid fear (needed a non-mol 15 months later).
My question to you is, has your DP ever given you reason not to trust his parenting?
If no, and this is a bolt from the blue, then yes, perhaps acknowledge you reacted strongly and talk it through/make it clear this is a deal breaker for you.
If yes, if you feel you are constantly on edge as you don't feel DP is safely in charge of himself and the situation either when roughhousing or more generally, then there may be a problem. Do you feel sitting down and discussing it with DP will be useful, or pointless? There are huge clues in how your DP reacts to this.
I think the thread got a bit derailed over calling a child 'good', who had just kicked and thrown something at an adult. Did you mean to say he's a normal 4 yr old, not a child who often kicks/throws? My DC was not perfect, but he didn't deserve what happened to him either. It can really mess with your head.
I do think it's a little more complex than it may appear to some posters, but I could also be seeing stuff that isn't there. Hope you can figure it out.