I think you sweat those 3-4 lbs. You may be underestimating the amount of water which you can get rid of by sweating. Here is a simple experiment:

One day, before you go to bed, weigh yourself then take a long hot bath. Keep your body in the tub under hot water for at least an hour. Then wash your self up, drain the water out and stay in the tub until your body stops sweating. When you get out, weigh yourself again and see how much you have lost. I do that myself sometimes so I know you could easily lose 7 lbs.

You may say "I was sweating heavily during the bath but when I'm asleep, I don't see myself sweating". Yes, during sleep your sweat has been evaporating as it comes out since it was not too heavy. This is why you lost 3-4 lbs over 8 hours while in the bath you lost 7 lbs in one hour.

The outside surface of the body is very large. This is why an invisible amount of sweat ads up to a large amount of water.