4 weeks the same then 4lb weight loss

Hi all - does anyone suffer from this annoying thing as well!!?

I find that I stick to MFP eat back my exercise cals earnt - weigh in the same time every week no chnages on the scale on or off and then boom 4th week it shows a 4lb weight loss.

Its frustrating when the scales/inches don't move for 3 weeks but I keep plugging at it and it eventually pays off - a rate of 1lb a week is sensible I suppose.