4bbl in a 5bbl fermenter?
Our brewery, 2 BBL brewhouse, is looking for a 4 BBL unitank fermenter but we can only find 5s currently. Will there be a problem frementing 4 BBL in a 5 BBl uni?
Probably not a problem. The issue with oversized fermenters is usually that beer doesn't reach the cooling jacket, but since you'll be using 80% of the fermenter, you shouldn't have problems with that...
This sounds like an ideal tank for your needs as it will give you the appropriate amount of headspace for fermentation.
Using a 5 bbl. uni. might be a good way to up your production a bit. Since you'll have to double brew to fill the 5er, try to increase your cast-out wort by 1/2 bbl. per batch.
You may also want to consider a high gravity brewing option - akin to what an extract home brewer does. The typically recommended method is to reconstitute AFTER fermentation, but I'd try it in the fermenter - beating the potential high ester problem seems advantageous in this method.
If you don't mind experimenting, that is.