This is great advice. 4 hours in Amsterdam will go by very fast. I'd keep it simple. Rent a bike, hit a coffee shop, go to the Van Gogh Museum, and then depending on how much time you have cruise the red light district and get a comfy snack.
If you are experienced with pot, I'd recommend splitting a space cake with a friend before you get on the plane.
But that's just me. Others would think that's terrible advice. in my world . . . Flying high while high sounds awesome.