1. Fold.

2. Call it, then if you do hit your nut flush/two pair/set and these guys carry on betting and calling you stand to win a huge pot. Then if you don't hit anything you can gladly fold to a pot sized bet.

3. Bet around $4 again, then if he makes a raise you can fold it.

4. Gonna have to call that, there's too much in the pot already.

Of course these are only my opinions!