How do I add a 4th disk on my RN214 that I originally set up with only three?

Dave_

Welcome to the Community!

There are few ways to add disks/Expand volume to your RN214. Please see Expanding ReadyNAS

One way is the method you used wherein you factory reset the NAS after backing up the data, this will create a fresh set of RAID configuration consists of now 4 disks on your NAS. The other option is to hot swap the 4th disk, this will do the same resync you are getting but it is only for the additional disk. the RAID is adding the 4th disk to the configuration. 

Regarding the size, do you have 3TB drives? If you have 4 3TB drives and factory defaulted the NAS, it will be setup as X-RAID RAID 5 where it will have 8.16TiB Data and 2.73TiB allocated for protection. RAID 5 has a 1 disk failure protection. Are you trying to get all the disks to be your data volume? in this case you need to set it up as RAID 0 or JBOD. You will also have to use Flex-RAID. See Flex-RAID.

JBOD - This most basic RAID level does not protect your data from loss if one of your drives fails. JBOD is available only on volumes consisting of a single hard disk.

RAID 0 -  Distributes data across multiple disks, resulting in improved disk performance compared to systems that do not use RAID formatting. The total capacity of your storage system equals the capacity of the smallest of your disk drives times the number of disks. RAID 0 is available on volumes consisting of two or more hard disks.

Though this setup does not have any protection so any failure from the disks will have your data lost and possibly unrecoverable.

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You may want to check on the RAID Calculator

HTH

Regards