I have 5 NAS's from different manufacturers such as Qnap, Synology and Thecus. Thecus is not as good as Synology or Qnap, but they are ok. Their OS is not as full featured and I have had more issues with the Thecus then the other brands. I would spend a little more for a Synology if I were you.
with 4 disks no need for hot swap spare. I don't think you can RAID 6 3 disks with a hot spare. You can afford 2 failures on raid 6 with 4 disks. I'd say RAID 5 with that few disks as well and afford one failure with no spare. Just have an extra drive around and pop it in.
the N5550 is a 5 disk model, that was why I asked about spare.
After looking at it and hearing opinions I decided its 2 year old model and its not bad but not what I want.