Which plan is this you are on? And assuming it's a VLCD then I wouldn't recommend dropping down to 3 if the plan recommends 4 as you'd be loosing out on your nutrients and could start sending your body into starvation mode and causing damage to your muscles and organs.
Most diets have a bigger loss in the first week then an average if 2/3 pounds there on as the first week you loose water, fecal matter and glycogen which weighs quite a bit, but then once your body starts burning fat the losses are slower but more beneficial. Interesting fact for you: the average person hold approx 10lbs of bodily waste in their system at any one time. So, hence when you start eating less then you drop a larger amount of weight in the first week as your system empties and the food you are eating isn't as much to replace it.
Stick with it as some weeks you'll have a bigger loss and others will be about 2/3 pounds.