B.W.
If he's been seen by a doctor with no problems found, then at 4.5, he knows what he is doing. I would not be cleaning up his messes for him. I would make him change his clothes, rinse out his wet undies and pants, and make him wipe himself up and get new clothes. I would make him wash/wipe up the mess on the floor, and then make him wash his hands. A few times of taking all that time out of play (and make a big deal out of what he's missing by having to clean up his mess) and he will be taking himself in no time.
Be consistent, at 4.5 he should be bathroom independent. make him deal with the messes and you will see a change pretty quickly.