Awwww, poor honey!
Yeah, I've had those ba-bam water-weight gains overnight, and they are NOT NICE. Especially when you're glued to the scale for progress reports on your journey!
But remember to keep in mind to take the number on the scale with a grain of salt. Like everyone else said, it's obviously water weight-- no one gains 4 lbs of FAT overnight, in the same way that you don't LOSE 4 lbs of fat overnight. Salt is diabolical for making you retain water, and-- if you're anywhere near your TOM (time of the month), you could even be doubling-up on the water weight gain.
Green tea, lots of water (ironic, I know) and your system will flush it all back out. Don't let yourself get too upset, just know it'll go back down.