1. Fill the 3-gallon water container and pour it into the 5-gallon water container.
2. Fill the 3-gallon water container and pour it into the 5-gallon water container which already contains 3 gallons of water. This leaves 1 gallon of water in the 3-gallon water container.
3. Empty the 5-gallon water container and then add the 1 gallon of water from the 3-gallon container.
4. Fill the 3-gallon water container and add it to the 1 gallon of water in the 5-gallon container. Presto - 4 gallons of water!