When there's a sudden change in weight, like 4lb in a day, IMO it's pretty much impossible for it to be a "real" change.
i.e. I don't think anyone can gain or lose 4lb of muscle or fat that fast without using a scalpel.
So whatever it is, water, food, glycogen I wouldn't worry about it. Look for patterns over a longer time scale.
And yes glycogen is glucose stored in the muscle and liver, but you can't really build up those stores beyond 'full'. It's a lot more common for them to be near-full than near-depleted.