Die Hard with A Vengeance question!!!?

Ok, in the movie "Die Hard with A Vengeance"(the 3rd Die Hard movie) John McClane(played by Bruce Willis) and Zeus Carver(played my Samuel L. Jackson) must solve a series of riddles to catch a criminal basicaly. Theres alot more to the movie. Anyway, in the movie McClane and Zeus must disarm a bomb. The bomb can only be disarmed if you put 4 gallons of water on a scale. It must be EXACTLY 4 gallons, no more, no less. There is a fountain next to them, along with a 5 gallon jug and a 3 gallon jug. I've always wondered how they got 4 gallons of water using only these jugs. They didnt guess or estimate the amount of water(ei, filling the 5 gallon up with 3 gallons, then filling the 3 gallon up a third of the way, and pouring that in). If your good at math or seen the moviue, please help!!!!!