Inexpensive NAS for over 4TB backups - recommendations?

On the 102 you can set snapshots to hourly, daily and weekly by default, it's on the share settings and called continuous protection. I believe snapshots are done by midnight (mine are at least according to logs), so this won't affect the work hours productivity and will be transparent for users. You can give them access to snapshots via a shared folder or choose not to and keep the access in the web interface only. I believe you never lose while activating the feature unless you work with really volatile data (like SQL DB or VM).
If all your mac are saved to time capsules it will be an extra layer of protection since I believe everything is done locally then uploaded ? So I guess everything will be picked up by at least the hourly update of time machine. The only problem will be the time to recover, if you don't have enough space it could be too short and you may not have the date you want when the snapshots are regrouped.

My setup is simple, I got a time capsule for my mac, then I got the NAS with raid5. Then for important data I got 1 distant backup and 1 local backup.