[The Dude]

You should definately 3-bet here. He didn't put in a raise on the flop or turn, so it's less likely he has 66 or QJ, although those are both possibilities. His most likely hand based on the action is going to be 2-pair w/ an 8, A8, K8, or perhaps even 86. You beat all these.

For the sake of argument, let's say you must always pay off a 4-bet, and you always lose when he 4-bets. Also, let's say he 4-bets you every time you're behind. (Which is obviously worse than real-life). He's going to pay off your 3-bet with any made hand, so if you're ahead 2/3 of the time, you should 3-bet. (You lose two bets when you're behind and win one bet when you're ahead.) IMO, you're vs a lower 2pair hand here more than 2/3 of the time. Factor in the times when he 4-bets and you're ahead and when he doesn't 4-bet a better hand, and you probably need to be ahead only 60% of the time, which is even better for you.

Raise it up.