How does your body drop 4 lbs overnight?

water weight of course - but not sodium.

And you said didn't change diet so obviously not going low carb in your case.

Rarely does sodium retained water drop quickly - gain quickly yes, but most do not see it drop fast unless they went the other extreme and did like a no-sodium day. Even then, usually multi-day recovery from a high-sodium day.

Water weight from stress though.
Elevated cortisol can slowly increase retained water upwards of 20 lbs.
How many weeks of scale weight could that hide?

A body being under that level of stress though is telling - perhaps the body considers that level of deficit too great, and you finally eat a tad more on some days and it whooshes soon after.

Lyle has an article on that fact too.

1-2 lbs a week may be getting extreme for the body now. Be concerned that the body may start adapting - because if you don't purposely make the deficit smaller, it will attempt to by you moving and burning less.

deleted. Didn't realize this post was so old.