Adding water to fermentor

So I brewed NB Caribou Slobber yesterday and my volume came out to be about 4 gallons.. OG was 1070... Supposed to be 1051.

Do you all think it would hurt anything if I added some distilled water today after I have already pitched the yeast? (Was not in any shape to drive to the store yesterday to get it)

If the answer is no.. should I mix it up after I add it as well?

I've done this a handful of times to no ill effect. I don't bother with distilled usually - spring water is just fine for adjusting volume at pitching time, and a lot less weird. In your case, though, since you're looking to dilute beer that's already taken off, it's probably a good idea not to introduce oxygen (dissolved in spring water for taste.) Make sure to sterilize everything: fermenter lid, bottle, and such. Avoid splashing in the new water, and definitely don't stir or shake the fermenter - anything you stir it with is a risk of infection, and agitating your beer will encourage oxygenation (tastes like cardboard). Worry not, it will mix.

Thanks!!

When this has happened to me, I've usually kept the beer as-is. The extra gravity makes it extra tastier.

There's no harm in diluting it to the expected gravity though. I'd recommend waiting until fermentation is done or mostly done so as not to disturb the yeast by sudden changes in environment. Boil the water for 10 min or so to sterilize and drive off at least some of the dissolved 02. Let it cool to room temp, then siphon in. As usual sanitize everything.

Now that I'm thinking about it, there is something to be said for having 25% more beer. . . .