ReadyNas 104 behaves strangely after adding disks

At this point, your safest thing is to do nothing and wait for the sync to complete unless you have a complete backup and are willing to just stop it, do a factory default, and restore the data. 

I can see a possibility of why it would show two volumes, and possibly therr.  The original two drives plus 2TB of expansion from the two new ones would be one layer, and the remaiing 2TB of the new ones would be another.  When you cycled power, maybe it interpreted the second layer as a second volume, and possibly a third since the two drives weren't merged yet.  Once sync is complete, that should not be catestrophic to your data, though you might need to copy if off and do a factory reset to get the state right if it persisted so you can use the extra space.

Apparently, one of the recent changes in the OS is that it now picks up "dead" volume mount points to allow you to delete them.  But another user had something similar and ended up deleting the wrong volume, losing all his data when it had survivied up to then.  So come back with an updated status if it completes and you still have "phantom" volumes shown.

If the NAS looked like it was still crunching away on the re-sync (as indicated by the drive access LEDs), cycling power was a bad idea, even if it was not accessible via conventional means.  If RAIDar also could not find it, then maybe you had no choice.  SSH could have been the ultimate way to find out, if you have it enabled.  But that's water under the bridge, as you did not come here and ask for advise before cycling power.