ReadyNas 214 : Adding new 2x 8Tb disk to 2 x 8Tb existing hardware, but one disk not detected.

Reallocated sectors have nothing to do with the NAS - they are monitored by the hard drive firmware itself, and it looks like disk 3 is defective.  Though it is in the RAID array, it should be replaced.  I'd exchange it with the seller if you can, as the manufacturer will exchange it for a recertified drive, and the seller will exchange it for a new one.  FWIW, I swap out hard drives before they reach 50 reallocated sectors.  Though different folks have different ideas about what the replacement threshold would be.

I suspect drive 4 was DOA - since the NAS isn't detecting it at all.  Perhaps there was damage in shipment of the new disks??  

Generally I recommend testing the drive with vendor tools (seatools for seagate, lifeguard for western digital).  Nothing against HDTune, but I think the manufacturer (and the seller) might place more weight on the vendor tools.  Whatever tool you use might offer to "repair" them (which basically means reallocate any bad sectors the diagnostic finds).  But these are new disks; if they were mine I'd exchange them.