Without looking at the previous replies:

#1 - This is a tough one, but I'm pretty sure he has you. JT? TT? Q9s? They would be iffy calls on the flop (unless its Q9 of clubs). But on the turn, he knows you have a strong hand. He wouldn't bet $5 there if he didn't have you beat, since you're likely to call.

#2 - This is a much easier fold. AK, AQ, two pair, and a set beats you. Run.

#3 - Pot odds justify this call, even if they do probably have it. Unless he doesn't bluff at all, just chases and checks when he misses, then you could fold.

#4 - This is another tough one. Either he trapped you with JT, two pair, a set, etc., hit two pair or a straight on the river, or is trying to bluff you out since you checked. But, you're getting more than 3 to 1 for your money. If he bluffs at the river at times, you should call this. Kind of looks like he missed a flush draw, and is now trying to buy the pot.