???? about electrolytes oral solution

I was givin' a script for tablets that cost $40+ with ins but was told by pharmacist that the powder mix was cheaper. I called doctor and changed to powder. I called later to get directions on how to take. They said to start at 2pm and drink 8oz every 15 mins and be finished by 5pm. Which is 4 liters of liquids. Is this possible? Should have I gone with the tablets? As you can see I am now 11 months post op. And by the way the reason for the bowel prep is a colonoscopy. Have problem with severe reflux and the doctor did the endoscopy thing yesterday (said I have something called barrett's esophagus and that my opening to my stomach was too small.) When I did my family history and told them that my grandmother and great grand father both died of colon cancer he also ordered a colonoscopy. Any help is greatly appreciated. Thanks Tina N 289/164/150ish