RAID 5 is the best option for a four-bay NAS... Note that in a RAID array, all the drives should be the same size, or else the RAID will configure itself with the smallest drive installed, e.g. if you have three 6-tb drives and one 4TB drive, the RAID will configure the NAS as four 4-TB drives, wasting 2TB on each of the larger drives.
It is NOT recommended you use non-NAS drives. The reason is that the error correction timings are different on a NAS drive. A non-NAS drive may error out in the NAS even if it is perfectly fine because the non-NAS drive will not respond correctly to the NAS.
Note that the NAS can take a LONG time to configure itself when you use several drives. IIRC, my NAS took two days to configure the array. If you keep rebooting the NAS, it will start the configuration all over again. Just let it do its thing until it is done.
I have a comprehensive thread here
about QNAP NAS. You should go through it as it will answer most of your questions. Read the section about error correction on hard drives. This will make you think twice about using shucked drives.
Understand that a NAS is NOT the ultimate backup for your files. Far from it. While it is a big step in saving your files, a NAS can still crash, wiping out your data. While the chances of this is slim there are possible scenarios where a total NAS failure can occur.
You can have a power surge that fries two drives. That will result in a total data loss.
Or you can have one drive fail in a RAID 5 array. That usually is not a problem. You can replace the bad drive and the array will rebuild itself. But if a second drive fails before the rebuild is complete, total data loss. The likelihood of this occurring is extremely slim, but I can attest that it has happened to me. I had Toshiba NAS drives fail at the same time, that's why I stay away from them. I like to stick with WD Red NAS drives.
That is why I have two NAS systems. One is a backup for the other. The second NAS is an eight-bay configured as RAID 6, which allows two drives to fail before any data is lost.
Hope this helps...