Fill the 9-liter cup first, then pour in the 4-liter cup. There are 5 liters left in the 9-liter cup.

Then go, and there is still 1 liter left. Pour the last remaining 1 liter into a 4-liter cup.

Then refill the 9-liter glass, pour 1 liter water into the 4-liter glass, and pour out 3 liters, leaving 6 liters in the 9-liter glass.