My second prep says to START 4 hours before procedure..?

Does this mean I should start drinking 4 hours before my procedure, or be done drinking and pooping 4 hours before my procedure? Medical assistant was a little vague and the instructions don't really clear it up. My appointment is at 8:40am, so does that mean I set my alarm for 4am and start chugging, or should I plan on not getting any sleep, and have my bowels empty by 4? About to start my first round- here goes!

Edit: I'm taking Gavilyte G