Ooo! Ooo!
Probably wrong but I don't care.
Place the three gallon bucket in the five gallon bucket and fill.
Remove the 3 gallon bucket and your left with 2 gallons.
Empty the 3 gallon bucket and pour in the 2 gallons.
Make a mark on the 3 gallon bucket showing the 2 gallon level. Pour contents into 5 gallon bucket.
Fill 3 gallon bucket to the 2 gallon level again and dump into 5 gallon bucket.
Presto! 4 gallons.
