How add drive 004?

When I type this address into a browser 192.168.1.140 I see the Home tab of my PR4100 NAS. It shows 14.1 TB Free. I have been operating with 3 out of 4 hard drives in my NAS all 10 TB. Using Acronis I have a backup chain going. I installed a 4th drive of 12 TB. A process that took several hours took place. Finally the NAS was complete. Disk Status shows my 4 healthy drives all Good. My Raid Volume is 17.91 TB. But the home tab continues to show 14.1 TB Free. How can I make the PR4100 NAS include my new drive in usable space?

What was the Raid level when using 3 drives? What the raid level now?

Vol 1 … Raid 5 … 17.91 TB … Good. That is what it says now. I do not remember what it said before

For 3 years I have been using 2 Red plus and one Red pro drive all Ten TB inn my PR4100. I have backed up numerous times. It was running 24/7 for three years just fine. What I now understand is that according to WD advice I should never have been able to do that. A plus drive and a pro drive cannot cooperate, their staff seems to believe. I am trying to find out if I am able to add a fourth drive, a new Twelve TB Red Pro. What seems to be required is that I wipe out all my stored files completely and reformat all my drives in the NAS and start over from scratch in order to get the fourth drive to be seen. It seems necessary to abolish my shares and destroy the name of my NAS as it is known by my Plex server. I will have to rebuild my shares and reteach Plex and Acronis to see them. All the reformatting will take a number of days. I have written my question to the WD staff and received an evasive answer. What I seem to be learning from them is that they do not respect my mixing of Pro and Plus drives. They seem to be saying that I should not expect to insert a new 12 TB drive and see one single megabyte of increased storage space unless I buy more new drives and start over.