The "c" is for 1 cup. I used regular old oatmeal oats, 3 medium sized bananas, 2 breakfast tea bags, 2 pounds of dark brown sugar, cheap 100% juice. I sliced the bananas into thin disks and put them in a nylon bag with the oats.
While steeping the oats and bananas, I lost almost a half gallon of water. So you are correct, the 4.5 gallons of juice is to top up to 5 total gallons.