Take him to the pediatrician. Frequent urination is a symptom of bladder infection and diabetes. If you find it is nothing, you may want to watch the amount of liquid he drinks before engaging in a sport. Karate teachers scare some children
I am noticing he does drink alot for a 4 year old. It was a very hot day today and yesterday. I am writing down how many times he goes. I just call the pediatrician. Waiting for them to call me back.
I agree with Koukla - this sounds like diabetes, if he's drinking so much. Or a possible bladder infection, but that usually hurts.
Best wishes.