Your nearly 4 hours would suffice for a quick look around AMS. Keep in mind though that you might face a sizable security queue on your way back to the gates, so factor that in. Something you might want to consider, just to have a touch of what Amsterdam looks like, is a boat ride. Yes, it's mostly a tourist trap, but if you're squeezed on time, that might be the best way to have a quick look around. Amsterdam Centraal is really in the heart of Amsterdam: you might want to have a short walk to Dam Square, then head back to the station and jump on a boat: the pier will be on your right hand side.