[PoorLawyer]

Probably should just call 4th. You are already heads up here. If you catch a scare card a raise on 5th is a lot scarier. You are heads up with a 4 flush or a pair plus overcard (maybe with a 3flush) most likely so you are either about even or behind a lot of the time.

On 5th, if you call 4th, it is easier to fold. I think it is also more reasonable for him to think trips in this case. You are either well behind or getting there if this is a reasonable player. I would lean towards fold unless you think he is so tight that you can blow him off the hand representing a bigger pocket pair in the hole.