Why can't you take him in? Not everyone is meant to be a multi-cat house, but, you have four now...will one more really make a difference? And, if so, and I respect that, then I think you (rather than a shelter) should try to re-home him. The problem is the shelters are soooo over populated, understaffed, etc., to burden them ......that it might be 'easier' for you to do it? Ask friends, neighbors, etc., if anyone is interested.

Good luck with your decision.