No, it's not possible to gain 4lbs of fat in one day unless you ate (3500 calories x 4) at least I think so. Just a guess.
I'd say its water weight. I lost 4lbs yesterday and I'm guessing it was all water because I ate 1600 calories.
I agree with Meshellibel, drink some extra water to flush yourself out. I read somewhere that the body will retain extra water when the body is toxic, meaning that perhaps you ingested too many toxins in the foods you ate yesterday and your body is protecting itself. It happens to me all the time when I eat bad.