You have 2 jugs (and only 2 jugs), one can hold 3 gallons of water and the other can hold 5. You need exactly 4 gallons of water. Assume the jugs have no markings, you have no measuring equipment, and you have infinite water.

See below.

Step-by-step explanation:

You fill the 5 gallon up to the top then you pour 3 gallons into the 3 gallon jug, leaving 2 gallons in the bigger jug.

You throw the contents of the 3 gallon jug away then transfer the 2 gallons from the bigger jug into the 3 gallon jug. So you have 2 gallons in the 3 gallon jug.

You then fill the 5 gallon jug and pour 1 gallon into the 3 gallon jug, so you are left with 4 gallons of water in the 5 gallon jug.