[MRBAA]

The pot is big already, so even though you'll lose more often than you'll win, the overlay the pot offers makes doing whatever it takes to increase your chances of winning worthwhile. I bet out on fourth and take it from there. Now I may get checked to on fifth if I play it that way, in which case I'll take my free card. As it stands, you can't fold, raising is too risky into two paired door cards (although I agree the fives is less likely to have trips since he didn't raise), so you call. On sixth you see what develops, but it would take alot to make me fold before seeing the river.