[Andy B]

I would definitely bet fourth after the field had checked to me. I think I'd raise fifth. Maybe someone comes along for the ride. You don't know if you don't raise. When the guy hits open trips on fifth, it may just be time to throw in the towel. If he isn't full, he's probably got a low draw, in which case you might only get half of a small pot. It seems odd that he'd check a low draw and then bet out when he paired. Seems like he had two pair on fifth and has now filled. Also, if he isn't full, he can still fill on the river. Finally, he can see your four clubs. If folding on sixth is wrong, it ain't wrong by much.