Measuring four gallons with 5- and 3-gallon jugs

You have an unlimited water supply, an unmarked 5-gallon jug and an unmarked 3-gallon jug. How can you end up with exactly 4 gallons?

Fill, pour, empty, repeat. Fill the 5, pour into the 3 (leaving $2$ in the 5), empty the 3, pour the $2$ gallons into the 3, refill the 5, then top up the 3 (which needs just $1$ gallon), leaving exactly $4$ gallons in the 5. It works because $4=2\cdot5-2\cdot3$, an integer combination of the jug sizes.