I had a blow-out and lost about 1/4 of my wort.
Can I replace with water or just leave it alone?
This blow-out happened on the second day in the carboy. I replaced the airlock.
Best advice is to probably just leave it alone.
However, if you really want to get your volume back and have the materials on hand, you could potentially make a mini batch of the beer (thats enough volume to want to hop it) and dump it in.
How long has it been since the initial yeast pitch? I'm not sure you'd want to pitch that much wort on a beer that's already mostly fermented. It probably wouldn't be catastrophic but you might get a weird flavor profile.
I would not replace the wort, I would leave it as is, as this reduces risk of contamination, and does not dilute your wort.
You can add boiled and then cooled water, but it will reduce the gravity and reduce mouthfeel and abv of your final product. I would not recommend this.