Fill the 3 gallon jug, then empty its contents into the 5 gallon jug. Fill the 3 gallon jug a second time, then pour its contents into the 5 gallon jug until the 5 gallon jug is full. This leaves one gallon of water in the 3 gallon jug. Empty the 5 gallon jug, then pour the one gallon of water in the 5 gallon jug. Fill the 3 gallon jug a third time, then empty its contents into 5 gallon jug. This makes 4 gallons. SECOND SOLUTION.