New disc installed no more space?

Assuming you are using the default xraid:  If you added a 4 TB drive to your existing 2 TB drive, then there will be no space increase.

The ultra normally creates a raid-1 volume (with protection against single-drive failure).  Space is limited by the 2 TB volume already installed.

The simplest path is to get another 4 TB drive, and replace the 2 TB drive also.  Then you'll end up with 4 TB of protected data.

Your other option is:

(a) back up your 2 TB of data somewhere

(b) do a factory reset with 1 drive in place.  Choose "flexraid" from Raidar (the 4.3.8 version, I don't think the 6.0 version will work for this). Set it up as RAID-0 with only the one drive installed.

(c) Add the second drive, and select RAID-0 for that one also.

(d) create the shares you want (some on the "C" volume and some on the "D" volume).  Recreate any users, etc.

(e) restore data to those shares form backup. 

This is painful (however with 4.2.x firmware you would have needed the factory reset no matter what).  Although flexraid will let you create a single 6 TB volume, that is usually a bad idea.  If either drive were to fail, you'd lose all data from both drives.  The instructions above prevent that from happening.