If you’re happy about SSH and maybe invalidating your warranty you could try the alternative, and reliable backup solution I have posted on here. It means you can backup whatever shares or folders on your WDMC you want.
It’s posted at Alternative to Safepoint
In response to the rest of your question, it isn’t possible to limit the size of a user’s share. I’d stick with the 4TB MC, it takes a lot of filling!
RegardsNick
