X-Raid 2 Expansion doesn't take place

Here are your data volume RAID groups:
md6 : 2930252352 blocks
md5 : 1953500928 blocks
md4 : 2441879280 blocks
md3 : 2441879280 blocks
md2 : 2418326480 blocks

While this doesn't give us the details of the expansion history, it does tell us that over time you've had 500GB, 1 TB, 1.5TB, 2 TB, and 3TB drives in the system (as well as the 4 TB drive you have in now). 

These RAID groups total to the right volume size (~11.35 TiB), so the expansion problem is with the ext file system, not the RAID array itself.

Your NAS has two expansion limits: 

• A volume can't expand over 16 TiB 
• A volume can't expand more than 8 TiB from it's original starting size.

The first limit doesn't apply to you, since your volume size is well under 16 TiB.  However, the second limit could apply.  Please open expansion.log in the log zip file, and search for the first occurence of The filesystem on /dev/c/c 

The block size on that line should be the starting size of the array.  You can check the date also, and see if that aligns to the initial install (or the last factory default).

If you have hit that threshold, you'll need to back up your data and do a factory default with all disks in place.  Reconfigure the NAS and restore the data from the backup.  You could also convert your NAS to OS-6 while you are at it - it doesn't have these expansion limits.