On forums like this, you get people like me who are passionately against routine infant circumcision where a medical need for it is not in evidence. You also get those who are passionately for routine circumcision as a preventative measure for a problem that may never exist. You also get a sprinkling of circumcision fetish folk, who I am best not to discuss at all.
However, all of these varying groups can usually agree that if a 4yo has an issue, something needs to be done. That something is consulting a specialist. Nobody on this forum can really give you any medical advice on what course of action should be taken in your son's case. That is why your first and only course of action should be consulting a peadiatric urologist. A gp can give you a referral, but shouldn't be your end stop, they are specialists in general medicine, not paediatric urological issues. Also, reading all the pro circ or anti circ sites out there won't be any help, they may just confuse the issue for you.