ORIGINAL: twildasin

Well I would fill the 3 gallon bucket and empty it in the 5 gallon bucket fill the 3 gallon bucket again and then put in what you need to fill the 5 gallon bucket mark your 3 gallon bucket with the remaining water which would be a gallon dump the 5 gallon bucket out put the 1 gallon in thats remaining in the 3 gallon bucket fill the 3 gallon bucket again and dump it in the 5 gallon bucket and BAM you got 4 gallons!