It's more than likely just water retention. It would be basically impossible for you to eat enough fat or protein to gain a few pounds over a few days. Look back at what you've eaten and evaulate whether or not it was higher in sodium. Then look and see how much water you consumed. Any time I eat things that contain sodium, I need to up my water intake to off-set the retention-- even so, I still can bounce up a few pounds, but then they go away again.

My advice would be to watch your sodium consumption, be sure to eat your veggies (no matter how yucky ) and be sure to get plenty of water.

HTH!
Nikki

PS- I've been gaining and losing the same 4 lbs. for almost 3 mos. now. However, I have been losing inches the whole time. Have you measured lately?