Fill the 9-liter bucket.
Fill the 4-liter bucket with part of the 9 liters (now only 5), throw away the 4 liters.
Fill the 4-liter bucket with another part of the 9 litters (now only 1), throw away the 4 liters again.
Pass the 1 litter that is left from the 9-litter bucket to the 4-litter...