KTo, 4straight on the turn.

[car ramrod]

I would check behind, and call his river bet.

A bet is not bad, but I can't fold to a raise, he could be doing that with a worse two pair, or as a semi bluff. If we check this turn, he is going to bet the river a large % of the time. This way we still get a bb out of him, as he may be more scared of the 4 straight then we are.