At 4 years old it is possible that he is getting a lot of exercise and pressure on his knees, children do get growing pains .is it possible to lessen the training ?
His twin brother and his colleagues in the club have the same exercise frequency. Maybe he is weaker than them?
They may not have growing pains if it is that it may be a good idea to ask the Doctor what he thinks may be the trouble .,is he well otherwise ?