Not really, RAID is not a backup, so you need some sort of a backup for your RAID array, therefore the recommendation for 2 separate systems instead of one, since he already has 4 drives of each.
I would recommend that OP use 4 x 8TB drives as his primary RAID array, and the 4 x 4TB drives as a backup for the primary to save at least backup important data.
Also for the OS I would not suggest Unraid at all if he wants to go the Linux route FreeNas/TrueNas core is the only recommendation, mainly because Unraid doesn't use ZFS and uses Btrfs / XFS which has its own down-side. then again ZFS with the amount of RAM requirement he'd need at least 48GB of ECC Memory (FreeBSD recommends 1GB of ECC memory per TB of Storage).
So if OP is comfortable with Windows and not comfortable with using the CLI for everything, Storage Spaces is the way to go, there is an added cost of licensing as well, mainly because of the familiarity of Windows and ease of setup for things like LACP/ Link Aggregation, Storage Spaces just has better flexibility in terms of removing/replacing drives, and it is 100 times better in detecting and reacting to drive failures where FreeNas would just panic, I understand the reason why people don't prefer Storage Spaces due to ReFS being proprietary and a big black-box but it just works, and if you still don't want to use ReFS we can always use NTFS, if the network is entirely Windows then there is no point in adding the complexity of a Linux in the mix.
I am testing both storage space and FreeNAS ZFS this weekend, I will be happy to post benchmark results. I will be using a 240GB SSD for Caching on both. I've heard parity speeds are poor in Storage Spaces but Mirror is much faster, but adding an SSD helps speed up the system by quite a lot.
I have no issues with FreeNAS or UnRaid personally, I can use and setup both without any problems, but the problem arises when we have to troubleshoot things, and me being familiar with Windows for the past 15 years, I can troubleshoot basically anything wrong with it much quicker and easier than I can in Linux where I have to post questions and wait for responses online which can take up to a day or more for resolution, I might not be able to wait for that long for that.
And yes ECC is a must for any data storage, if the OP is not concerned about losing his data here is a great source on the reason to use ECC
Source:
ZFS on Linux vs Windows Storage Spaces with ReFS | by Brian Smith | @brismuth’s blog