I was also on the 4 week pre-op.
I went with the slim-Fast as it was the easiest in terms of both variety and not having to weigh stuff.
From the start of the pre-op to the day of my op I lost 8.1kg or 1st 4lbs.
I did cheat, but both times I felt very guilty. The first time was about 3 weeks pre-op when my son walked past me with his plate of fish and chips and I quickly snaked a chip of his plate. The second time was about two weeks pre-op when I ate 5 Pringles crisps.
I did find the pre-op diet to be one of the most difficult things I have ever done, even more so than when I quite smoking. I guess one reason must be that you can live without cigarettes but not food. What got me through it was telling myself that this was the first and most difficult step in the journey to a new me that will be healthier, fitter, thinner, and expect to live longer.
Trust in yourself and remember that if you cheat you are only cheating yourself.