Yes and No
I don't think it is possible to gain 4 pounds of fat in one day, however, thats not to say you might not increase in weight that much. I do every weekend, but that because of beer and sodium plaing havok on me and making me retain water. Another possiblity is that you are not "moving things" through the system as effectively one day to the next. Happens to me sometimes too where I am 3-4 pounds heavier for a couple days, then I have a very big "movement" one morning and 4 pounds are gone! A little disguting but true. Thats why weighing yourself everyday has to be taken with a grain of salt. The reuslts are soooooo varied and dependant on so many things that it really isnt all too accurate at the best of times. Give yourself a couple days and drink lots of water to flush yourself out. Might just be the ticket.
sirant