For a 4 yr old to be playing by himself is totally normal!! And he seems a bit young to be harsh about the problem. I suspect it would work itself out with kind and positive actions on the parents behalf. Not to say it doesn't need to be addressed and worked with. Ridicule will seldom bring good results-if ever! I do believe it disciplinary actions with firmness, but this could become a real struggle if not handled carefully.
I just read the above post and would agree that chiropractic care is something to consider. I know it has helped in many bed-wetting situations.