Alright. Duckshot's ducks are hot in the oven and it's time to make Grandma's secret gravy recipe. We need six cups of flour-
First I fill my nine cup measure and pour out four cups into the four cup measure leaving me with five cups in the nine cup measure. Then I pour our four of the five cups remaining in the nine cup measure into the four cup measure leaving me with precisely one cup in the nine cup measure. Now, I pour that one cup from the nine cup measure into the four cup measure leaving me with precisely three cups of empty space in the the four cup measure. Then I just fill the nine cup and subtract three cups using the four cup measure which has one cup in it.
As Grandma says, "Soup's on!"