ReadyNAS RN314 Degraded problem seems serious

Degraded means you have lost redundancy.  Your files are at risk if there is a second failure, but you have most likley not lost anything yet.  A backup and resync ongoing simultaneously is going to put a lot of stress on the drives, whihc could trigger that second failure, and each is likely to slow down the other.

If it is drive 4 that is re-syncing, then you best bet is to remove it and stop the re-sync, as it likely will fail anyway.   But it's important to know if it is drive 4. If it's not and you pull 4, that could make it worse. Can you download the logs via RAIDar?  If so, mdstat.log will tell you what drive is in resync.  If you can get in via SSH cat  /proc/mdstat  will also tell you.

 But if you can't get into the GUI or download the logs from RAIDar, you don't know for sure if the problem is drive 4.  You can try pausing the backup and trying the admin GUI or RAIDar if nothing above works for you.  Everything else I can think of requires a re-boot, which is not a good idea before your backup is complete.