My parents thought I was a truly terrible, horribly behaved child at age 4. My "crimes" included things like acting up while we were in line at the bank, whining for snacks or toys, and once in a while saying a naughty word (which I'd heard plenty of from my parents).
My mother decided to give me not coal in my stocking, but another item she had told me Santa gave to bad children. When I came down on Christmas morning and saw the item, and no gifts around, I wailed and cried. After a few minutes my parents laughed at me, saying, "I think she's learned her lesson," and got out the rest of the presents.
Even today, the memory of the way they acted makes me so upset. It seemed sadistic -- they knew they were going to give me my gifts anyway, they just wanted to make me cry some beforehand. It wasn't the first or last time that their behavior was like that (pretend to take away something so they could give it back after I'd become hysterical with sadness and remorse). When I hit a child at around the same age, they also took me to the local jail and told me I'd go there if I didn't start being good. I had nightmares about it for months.
What's worse, to this day they bring these incidents, including the Christmas one and the jail one, up as an example of their good parenting skills.
If you do decide he's getting coal which I don't think you should do I think he should be made well aware of it ahead of time. What cruelty, to make a child anticipate and long for Christmas...I remember barely sleeping on the night before, dreaming of the lit tree and presents...and then take all that anticipation and joy and twist it into a spectacle of grief.
Remember this: it's likely your child will remember your behavior about this Christmas, if you actually follow through on your plan. He might forget a normal Christmas, and he'll almost certainly eventually forget the physical aggression he's directed toward his classmates. But the odds he'll forget coal in his stocking? Nil. And someday you're going to have to tell him the truth about who really bought that coal.