"Where I am from a child would be removed from your care if it came out that you hit them, regardless of leaving marks or not. It is honestly not OK to treat people like that."
I was smacked as a child.
Considering the outcomes awaiting children in care, and the trauma of being removed from their family (much worse than the momentary shame of being smacked), thank god no one had the brilliant idea to remove me from my family.
"Protecting" children by inflicting a much greater trauma on them, and putting them in situations where they're extremely vulnerable, is not actually helping children.
I shudder at the thought of the damage that well-meaning fools like you would have caused me if they'd seen me be smacked (I must point out it only happened when I'd behaved abysmally).
Abuse is inflicting long-lasting and/or significant harm. Punching a child in the face is abuse. Smacking their bums isn't. I'm sure most sane people can tell the difference between the two.
There's also a lot of people comparing this situation with a husband hitting his wife (but conveniently forgetting the part where the child was harming both his sister and the OP). So let's compare :
Wife throws heavy object at husband's face, giving him a black eye
Husband smacks her bum a few times, not leaving a mark
Who perpetrated domestic abuse (i.e. caused significant long-lasting harm) ? Go on, I'll give you three tries.
Now can we go back to the real world rather than compare situations that really can't be compared ? No, smacking a child isn't great parenting, and OP should take steps to not do it again. She knows that, she doesn't need to be drawn and quartered repeatedly. What she needs is tools and advice, no need to throw in the sanctimonious, holier-than-thou, gleeful finger wagging. I'm not suggesting we should congratulate her and say there's nothing to worry about, but surely there's a middle ground there (that only a few posters have found, unfortunately).
OP, I stand by the advice I gave earlier, you need to manage your son's behaviour before it reaches this point. If the advice on here is insufficient, I'd suggest looking for professionnal advice on how to manage poor behaviours.
Personally I haven't found ignoring (which someone else suggested) to be helpful. Either the behaviour IS attention-seeking (and then it will just escalate until you have no choice but to react), or it's not (and then by ignoring it you're giving the child free reign to do as he wishes).
I think strict management works best (so don't wait until he's totally out of control, intervene as soon as the undesirable behaviour appears). This way you're handling the situation while it is still quite minor, and while you are still calm (and the child hasn't worked himself into a frenzy yet).
At 4, for him to understand, the reaction must be immediate (else he won't link it to the offending behaviour). Time out works great for me but it can also be removal of a toy (if he throws anything at you I'd remove that object from him for the foreseeable future) or the end of an activity (if he can't draw nicely or sit quietly in front of the TV, then it's the end of the drawing/watching TV time).