Acquire the 7 gallon vessel.
Make it full.
Pour the 7 gallon vessel into the 4 gallon vessel.
Acquire the 4 gallon vessel.
Make it empty.
Pour the contents of the 7 gallon vessel into the 4 gallon vessel.
Make full the 7 gallon vessel. Pour its contents into the 4 gallon vessel.
Place the 7 gallon vessel on the pedestal, for now it holds 6 gallons.
It is done.