[bigredlemon]

On 3d, I'd put him on either 3 low cards, split 7s + a low card or another low pair, or 3 suited or connecting lows. Given the crappy door cards behind him, a pair bigger like QQ+ would raise here to get it HU, and 99 would also either raise and try to steal it right there or just safely fold. 8, T, and J pairs are unlikely given the cards out.

4th: his first bet could be a steal, it could be a 4flush, or it could be a slowplayed high hand. I think you have to either raise or fold here, and raising is more fun than folding. You could buy a freecard to draw to your low/flush, you're giving him a chance to fold, and you're setting yourself up for a steal if you catch well on 5th.

A raise on 4th also makes 5th street easier to play. If you both brick and he bets out again (even if he didn't raise you on fourth, and moreso since he did), you can safely throw your hand away. If you hit a low and he bets, you can raise again. You'll almost never get raised again here, and he'll definetly check to you on 6th no matter what he hits, thereby giving you control of the action.