I also have a boy that is four and loves Karate. He doesn't go with me to the dojo yet as the age requirement is 6. Sounds like our two boys are alike. I currently work with my boy at home. He has his own Gi and he puts it on and we train. My only concern with starting them this young is that they have short attention spans. Not that my boy gets bored with Karate, it's just that after about fifteen minutes, he wants to get crazy and come at me and try out his moves. I am going to gradually start making it more structured for him. For example no sparring or trying out his moves until we work some techniques. My Sensei is also a good friend of the family and has told my son that when he learns the first two moves of his kata he will award him with one patch and another when he learns the full kata. Personally I think my boy is ready physically and is very interested right now in karate. My only concern with him going to the dojo with me and taking an actual class is he may get bored because of the structure. By working with him at home I hope that he will learn to listen to instruction and have a longer attention span when he is the right age to attend.