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I always raise this. More often than not he'll be check-raising a set here. This looks like a pair of 4s that picked up a flush draw or more likely a 7 not wanting to give a free card.

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I agree with the raise. The other possibility is just someone taking a shot on a scare card. 1.7AF, playing nearly half his hands is pretty aggressive. We still have an overpair.

The more interesting situation is when/if he 3bets. I probably call and see what happens on the river. If he just calls the turn and bets out again, I should probably fold, but will probably call.