What are the Time Capsule performance if used as a File Server for 3-4 clients?
The drive is a nice, fast server grade drive but the controller is underpowered to be a NAS for professional video work.

That being said, even with two people reading / writing lots of small files, the Time Capsule works surprisingly well. At times when 3 or more people are hitting the drive at the same time for small IO, you will certainly see a slow down. Much more so than on a real NAS or server that has RAM to cache the filesystem and not be limited to the physical disk access.

So if the usage is low or you are on a tight budget, have at it!

Do be sure to have one of your clients be responsible for backing up all data stored on that volume since most backup solutions do not read from a Time Capsule directly. Using the built in clone tool is not at all optimal for production use, but is fine to make a snapshot over night or over a weekend when you have occasional cause to snapshot the data.