Oh, and to clarify, you'll need to download the DNxHD codec from Avid's website to export from Premiere with all available options, but it's free and it's the official codec.
ProRes is popular for this sort of thing, but it gets a tad murky on Windows, especially once you're talking about 4K. Premiere can import ProRes natively on Windows, but it can't export it without third-party codecs from those such as Miraizon. I've seen lots of stability issues using Miraizon with 4K and wouldn't currently consider it stable for archival under Windows.
If you're archiving 1080p, DNxHD is always a good way to go. If you're on a Mac, ProRes is great, be it 1080p or higher. If you're on Windows and want to archive 4K, it's currently tougher. We've been actually using Cineform for the moment. Hopefully DNxHR is released soon, which should finally give us a solution for this.
Without knowing more details... If you're just looking for something that works reasonably well, has some redundancy, and largely runs on its own, Synology's DiskStation products are pretty good. We have several people who edit over LAN with assets on our DiskStation, and have no huge issues, other than the limitation of gigabit.
There are better solutions out there, but I can't recommend too much without knowing your budget, level of sysadmin experience, etc. Building a PC that holds a bunch of hard drives and putting FreeNAS on it is a fantastic solution. ZFS, even better.
If you're used to editing over Thunderbolt, gigabit ethernet is going to feel pretty slow. Hopefully you're not editing anything higher than 1080p.
More generally, keep your footage always in a RAID6 configuration for safety, stay away from Drobo products, and periodically back up everything to bare hard drives, regardless of how awesome/redundant your NAS is.
If you are editing on Adobe, you are getting more flexibility and bang for your buck on a Windows desktop. Period. We can all argue about stability, but both platforms are very mature and both are also imperfect. Windows PC hardware, though, is cheaper and more customizable. I do still recommend Mac laptops though, as I do still think Apple wins there.
There's no such thing as "no crashes ever". Unless you're extermely lucky, or not pushing your hardware/software towards its limits.
If you're on a Final Cut Pro workflow, then stay on the Mac! If you're on Avid, my understanding is that it's a tad more stable on Windows.
Z820 will absolutely be great for editing 4K natively. Just make sure to put a really good graphics card in it, such as a Titan Black. (No need for the Quadro K5000 that comes with some of the Z820s.)
Cheapest, easiest, best thing you can do? Get an external Thunderbolt or USB 3.0 drive dock and buy 4TB bare hard drives. Make sure all footage is on 2 of them; plenty of these docks will hold 2 drives at a time and some will have standalone duplication functionality.
Don't be fooled by expensive storage systems. Just make sure you make 2 copies on set.