Raise 4th.

Calling 5th is probably OK because he's the bring-in. The odds on him having a pair in the hole or a two in the hole are quite long, although shorter than 16:1, given that he didn't fold 4th. With a lot of junk eg 7322, he should fold.

If you raise 6th, you should bet the river. Otherwise just check and call. Bet the river if he checks.