4 yr old doesn't play.

[torontokids]

Sounds like Aspergers (typically super smart, they struggle socially and become fixated on certain things e.g. angry birds, trains, star wars). You mentioned how he is smart and how gentle he can be. Can you put him in more of a leader role now and have him initiate play with the little kids? This will also give him a chance to practice those skills as well (turn taking etc) but in a teaching role which will still reinforce this for him. I worked with one kid who was obsessed with star wars and angry birds. It was definitely annoying but the other kids looked up to him and he was quite creative so he would lead them in "star wars" games outside where he would train them to be jedi knights which was pretty cute. I also bought a few star wars books etc and gave him special privileges e.g he was allowed to play in the gates kitchen as he was big and he could work on big kids things without interference.