1 pound is about 3500 cal, 1 kg is about 7700 cal.
For healthy, slim, shorter people, youre likely looking at about a 1500 calorie expenditure doing nothing each day.
Even if youre doing intense exercise, and can maintain it the entire time, youre looking at 400-500 calories per hour.
Weight fluctuations can be as much as 5 pounds thanks to fluid retention and whatnot.
...So even considering all the above, 8.8 pounds/4kg is an absurd amount to lose in 1 night. Not unless they drained all fluids, expelled all waste from their systems, and maintained elite athlete tier of performance would they come close.