Smaller batch

Anybody use a 5 gallon carboy as a primary. I have a 6.5 primary and a 5 gallon secondary. After reading this forum for a few months I decided to just leave in the primary for three weeks and skip the secondary. So now I have a empty vessel just sitting there.
I want to use the 5 gallon as an additional primary and make some experimental recipes. My question is, would a four gallon batch leave enough space in the five gallon carboy for the krausen? I've noticed that every beer I've made, four batches, has a krausen that behaves uniquely. My pale mild had a krausen only about 2 inches whereas my porter overflowed.
Any thoughts.
Thanks.

It probably would- but as you said each krausen behaves different. I noticed that some beers, like wheat beers, usually get a much bigger krausen and more explosive activity.

You could try a smaller batch, and use a blow-off tube if you need to. You could also try Fermcap-S in the fermenter. I've never used it in the fermenter but I've used it in the boil to reduce foam.

The small batches I have done had very small krausen when fermented between 62 and 64 degrees. That might work for you.

I am in the same situation. I have a couple of carboys that I very rarely use, since I started leaving everything in the primary for the full time.

I recently decided that I could use those carboys for smaller batches of beers that may take longer. This way I can use the carboys for those brews and not disturb my pipeline.

I measured out 4 gallons and put it in my carboy and marked a line on it. I recently brewed a Belgian Tripel in the carboy. I did have some blow off because the 3787 yeast went crazy, but 4 gallons seems about right for most beers in a 5 gallon carboy. I would definitely recommend a blow off tube ( I pretty much use one for every brew)

Now I trying to decide what to brew to put in my other carboy.