Hi archantgirl, great going on your weight loss.. I think a weight loss of 3-4lbs per week is probably a good target. Then, if you lose more some weeks, you will feel extra proud.
One thing though. I dont think you should be consuming so much water. The cambridge diet says that we should consume 2.5 litres of fluid (not including the water used to make the sachets). This is 4 pints. You said you were drinking 4 litres.. that is quite a lot. The recommended amount of water per day is 2.2 litres for women, or 3.0 litres for men. Drinking more than this can be dangerous, and evidence of this can be seen in recent news were a woman who was trying to win a contest drank a large amount of water and died from water intoxication.
If i were you i would cut down the intake to the recommended 2.5 litres, but other than that, great going girl! You will be at goal weight in no time...