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#1
I haven't used RAID in a long time. (I once used RAID 10: 4 identical disks, with the total capacity of 2 of them.)
RAID 5 effectively has one redundant disk. (The parity information is distributed, so all disks are equal.) If one disk fails, the remaining disks contain all of the data.
That means that if you have N identical disks, the capacity of the array is (N-1) times that of a single disk.
If you want 8TB from your 4 disks, you'd have to go to RAID 0. That's not recommended, as there is no recovery from a failed disk.
https://en.wikipedia.org/wiki/Standard_RAID_levels
You should have had 4TB (3.725TB in Windows) with the 3 disk RAID 5 array.
Go to Settings/System/ Storage/Manage Storage Spaces. Or Control Panel/Storage Spaces.
Create a new Pool and storage space
I studied the first post again. I can see that I might have concluded that the capacity of the array as seen by Windows was unchanged, but the language wasn't as obvious as it could have been. My apologies. I couldn't understand how someone could use RAID 5 without a zeroth-order understanding of what it does.
I have never used Storage Spaces, but it appears to be a Win10 means of doing software RAID. I'd be surprised if you could switch from the Intel soft RAID to Storage Spaces without breaking the Intel RAID (and losing the data on the disks).
Sorry, I can't explain how Disk Management sees the correct capacity while Windows 10 does not.