4kgs is a lot of weight to loose in just one week... Are you sure that's correct? Do you weigh yourself at the same time of day? First thing in the morning, after having a wee, is the best time to get an accurate weight.

If you are sure it is 4kgs in a week, I would probably be talking to my care provider to make sure that weight loss that is that dramatic is ok. You may need to look at your diet... Of course, I'm no expert, but for me I tend to think that any more than a 2kg (at the most) in one week, especially when pregnant, is cause to be concerned