Answer
Fill the 5 gallon jug.
Pour the 5 gallon jug into the 3 gallon jug.
Empty the 3 gallon jug and pour the remaining 2 gallons in the 3 gallon jug.
Fill the 5 gallon jug and pour 1 gallon into the 3 gallon jug (remember it will only take one gallon)
Viola the remaining 5 gallon jug will have exactly 4 gallons remaining.
