~ Had 8 cameras and a 4GB hard drive. Got almost a month of saved vision from memory. ~
Assuming you mean 4TB?
The Synology setup I have only records when a trigger event occurs, plus it back-captures a specified amount of time before the event (I have mine set to 10 seconds) and continues recording a set amount of time afterward. I have all my recordings going to a dedicated 8TB drive that rolls-over un-tagged recordings on a FIFO basis. I haven't actually checked but I think I get quite a few months worth of video before it starts recording over old stuff.
It would be much longer if we didn't have a mob of kangaroos and a bloody 'pet' horse triggering (no pun intended) the cameras all the time!
My NAS has gotten a little outta control though, so far I've added the expansion unit with five more hard drives (for a total of nine in this NAS) and I've currently got 120TB worth of storage... and I'm starting to wonder what I'm gonna do to expand that again soon!
Granted most of that is taken up by media (movies / TV shows / etc.) but at $800+ per 16TB hard drive it's getting to be a rather expensive hobby!