Four Good Measure

Deep in the Grey Labyrinth you have discovered a bowl filled with the Water of Truth. Legend has it that drinking exactly four cups of this illuminating liquid will confer great wisdom and reason upon the imbiber.

But drink one ounce too little and you will lose all wisdom and reason you had to begin with.

Drink one ounce too much and you will gain a week long hangover.

Beside the bowl are, inconveniently, two vessels measuring three and five cups respectively.

Do you have enough reason to measure out exactly four cups without wasting the precious Water?