Where is all my NAS Capacity? 4TB of Data and only 550GB Free on 4x4tb Raid 5 NAS

So, the subject line says it all.

I have a very well functioning NAS (EX4100) with 4 x 4TB hard disks installed on a RAID 5 setup. I have a total of about 4TB of data on the system and the GUI is telling me I only have 582GB free.

My firmware is up to date (2.31.204) and all 4 drives are showing as healthy, as if the RAID. I did find about 1TB of deleted files in the Recycle Bin, so that is a little of it. But this thing should have atleast 10TB available when new so I should have at least 4-5+TB available now, right?

What am I missing?
TIA

You are correct. So much for everything being Healthy. What is the point of the NAS saying everything is healthy if it is not. This is the error I got with the disk scan:

File System Check Failed
File system check has detected errors on your configuration. Backup your data and recreate your drive configuration to resolve these errors. If problem remains, contact WD Support.
Monday, 2020 February 17, 5:17:52 PM Code:36

I’m a novice at this stuff. What can I do to fix this?

Thanks

Your response is not helpful. Please let someone that is willing to help respond to my question.

I guess when you buy a car you never take it in for repairs as you clearly know everything about everything and can fix anything and everything yourself. Not all of us are blessed with such an overwhelming knowledge of everything thing ever created that we don’t need the help of others at some time.