Do you have any other empty kegs? Assuming it's a 5-gallon batch, what do you plan to do with the gallon of new beer you can't fit into the currently-used keg?
There was a thread on here a while ago about people not cleaning kegs, i.e., racking new beer into a kicked keg. I thought for sure you were going to ask about that. Full disclosure: I did that about 3 months ago with one batch; there were no discernable negatives to that one instance.
If you rack 4 gallons of uncarbed beer on the 1 gallon of carbed beer, the overall result will be very much undercarbed. You'll have to force carb it to get it ready for tomorrow.
It's not at all uncommon for people to blend beers; essentially that's what you're proposing here. If you force carb it in the meantime, no reason I can think of why you'd have issues, though you'd still have a gallon of extra beer left.
Occasionally I'll have a gallon or less in a keg and I want to clear space for another; I'll bottle that last bit of beer, pull the keg and now I have room--plus some bottles of beer I can share or take with somewhere else. I bottle off the faucets using a growler-filler, it's pretty easy.
