Hmm, if 2 and 3 are wearing the same color hat, 4 knows he has the other color.

Now, it depends on the type of hat. If it's something like a cowboy hat, then they can see the brim.

I know the solution to the second one, though. Light both ends of one rope and one end of the other rope on fire, when the first rope is done burning, light the other end of the second rope. The first rope when lit at both ends only take 30 minutes to burn. After 30 minutes of burning from one end on the second rope, you are left with a 30 minute rope. When you light this rope on both ends it becomes a 15 minute rope.