4 Gallons into the 9 twice to equal 8 in the 9 bucket...now fill the 4 again and top off the 9 which leaves 3 in the 4...now dump out the 4 and fill the 4 from the 9 which leaves 3 in the 9 now dump out the 4 fill and add to the 9 this makes 7 now fill the 4 and top off the 9 this leaves 2 in the 4