|Are you actually looking at the BIOS drive configuration screen and then the POST screens at start up? |
Watch the POST screens. All 4 drives should be identified by BOTH the model and the CORRECT capacity.
Many versions of RAID combine the storage of multiple disks. In that case the capacity would be the total of all the drives.
What settings are you using in the BIOS for the drives?
BTW, if you are simply seeing an Asus logo at start up hit Del and enter the BIOS. There are settings to disable the logos. There may be more than one. Then the POST screens will show.
It isn't clear if all 4 individual hard drives initially were showing or not.