This sounds like the 2-4 on paradise, which I've played. From my experience, it's almost certain he had the quads on 4th. Look at it from his point of view: dealt rolled, he just brings it in for deception. Darn, only one player stays and he's possibly on a steal -- don't scare him off with a re-raise on third. Now on fourth you catch quads. No need to worry about free cards -- you will almost certainly win the pot. Your concern is in keeping your opponent in, and he sure looks weak. Hence the checks -- you are hoping he makes a hand. Unfortunately for you, that's what happened. I'd actually hesitate to even bet sixth with a str8 given how small the pot is -- but would probably do it since opponent will call with two pair. Once you're raised, it's likely you're either beat by a made full house or he has trips and is drawing to beat you. I would not reraise, but I have seen players get crazy with two pair, so I'd call the river.