Hi, Smash ! Thanks for commenting on my blog ! Don't let that 4 pound "gain" get you down. it's probably nothing more than a temporary water weight gain. I can easily gain 3-4 pounds in a day ! it's not a fat gain. it's not a muscle gain either. not overnight.
You're going to see many fluctuations in the scale and that's perfectly normal. A big increase or decrease is almost always water related especially when it's really hot out like it's been.