How to add the 4th HD to mycloudex4 NAS Drive

Hi, I have tried searching for information about adding a 4th drive but I can’t seemed to find a solution here… lots of questions but no answer. Can someone help me with this?

I have the MyCloudEX4 with 3 4TB HDD installed as RAID5 (a holdover when my Mac Pro died). Bought another 4TB drive recently and wanted to add it to the NAS. The drive is already in and it is recognized as a new hard drive… but how do I format it and use it? The format option on the interface only shows 1 Volume but not the new drive.

I do not have any excess drive or capacity so am unable to migrate all my data out of the 3 4TB drive to format them all and them move them back in again.

Any help and assistance with this is greatly appreciated.

Ive the same issue… No answers anywhere