I haven't had Klean Prep, 4 litres sounds a lot to shift. The stuff I had was Movi Prep which has two sachets each made up with one litre. They were quite large sachets, about the size of a packet of savoury rice, so I'm thinking that there was tons of glucose in them too. I certainly didn't feel hungry or faint like I expected.
I had Pico Lax sent with a previous appointment, which was different, those sachets were about the size of the tomato sauce in cafes, so I got the impression that perhaps they were just laxative without glucose. I didn't use those in the end, so I don't know whether I would have felt any more hungry.
I agree with Mishap, don't go against the instructions, if the bowel prep isn't good enough you will end up having to do it all again (that happened with me for complicated reasons I won't go into). Remember to take some sandwiches with you for when you come out, because you will probably have to hang around for the sedative to wear off afterwards before they'll let you go.
I found that it wasn't anything like as explosive or urgent as I had imagined, so there was no real rush for the loo. Once your cleaned out bear in mind that anything you drink will run straight through in a matter of minutes, though.