You would need 1 quart of cleaning solution.
This is because you need a gallon of window cleaner, right? This is made up of 1 part window cleaner and 3 parts of water. 3+1=4 parts total. 1 gallon / 4 parts = 1 quart. So each of these "parts" is equal to one quart of liquid. Therefore, you will need to use one quart of cleaning solution.
