matt lynch’s review published on Letterboxd:
Fill the 3 gallon jug. Pour those 3 gallons into the 5 gallon jug. Fill up the 3 gallon jug again. Fill the 5 gallon to the top from the 3 gallon, leaving one gallon of water in the 3 gallon jug. Empty the 5 gallon jug. Pour the remaining 1 gallon (from the 3 gallon jug) into it. Fill the 3 gallon again, add that to the 5 gallon jug. Bingo, 4 gallons of water in the 5 gallon jug. Thing is, that's not how they do it in the movie, so it's confusing.