WD my Cloud PR4100 RAID 5 inquiry

Hello, I try to setup my NAS device WD my cloud pr4100, I have 4 TB *4 drives
As I know if I use RAID 5 it will use the 3 volumes and disk 4 could be used as hot spare, so I will have the capacity of 12TB to use + 4TB as hot spare
“the above is also clear in the manual that in RAID 5 it uses quarter of each hard disk”

BUT while I’m setup the device to be RAID 5, it shows me that Volume 1(1,2,3,4) are = 12TB, and if I check mark to use the “spare” option the volume changes to be (1,2,3) and the capacity become 8tb only ??

any one can explain to me where I go wrong !!!