Weight gain this early on?

So I've always been anal about my weight and am used to weighing myself a few times per day. I just got home from work and I'm weighing in at 4+ pounds heavier than I was this morning!

I drank quite a bit of water today and didn't pee as much as I normally do. Is this actual superquick weight gain, or am I just retaining water?

I'm 4 and a half weeks pregnant