Is raising on 4th street really that bad of a play? An argument could probably be made that he may be looking at a couple bricked 3-flushes, and that raising prevents these players from trying to backdoor their hands. It also increases his chances of winning with two pair or trips, and gains a free card on 5th street when Seat 8 makes the expected check.

Checking 6th seems like the best option. A bet is going to be made regardless of whether or not he checks or bets, but he at least he prevents the possibility of getting raised by checking.