A couple of things spring to mind, actually.
First of all, there's absolutely no need to force yourself to drink 4 litres of water per day. The minimum fluid intake is 4 pints which works out at around 2 and a half litres (just a bit less, in fact). That thing about the more you drink, the more you shrink just isn't true. So don't feel that you have to drink 4 litres, okay?
As Lexie said, you could try drinking peppermint tea, decaffeinated coffee, etc. This all counts towards your minimum 4 pints per day.
Secondly, 4 pounds is still a great weight loss. Yes, it doesn't seem great compared to some of the first week losses posted here--but that'll be because many of those posting those huge first week losses weighed considerably more than you do to start with. Bear in mind too that it could be that it's your time of the month, or that you could be a bit constipated--and that tomorrow, you might find you've dropped 2 or 3 more pounds overnight. That's how this diet seems to work.
I'm not so sure that the water flavourings have very much to do with it. Yes, they're reported by some to slow things down, but they never have for me and besides, overall, your calorie intake has to be way down on what it was. If you came straight off another diet on to Cambridge, that first week loss might not be as great either.
Lastly, don't worry about the ketostix. Some people lose five, six, seven stones or more and never see those pesky sticks turn pink... If you're sticking to Cambridge (with or without water flavourings), the weight will come off.
Chin up hon. You're doing just fine.
