Well, your failure of two of four drives after 3 years would in my mind be considered early, but you still would have had about 24,000 hours on them. Keep in mind that this was 24/365 use, far in excess of what one would see if they were in a PC for example. My one failure in the first months would be considered premature, but some in this forum believe that many of the premature failures, especially in clusters are possibly the result of rough handling during shipping.
It is not a question of whether a drive will fail or not, it's a question of when. In reality, I have had better service from my drives in the NAS service from a pure standpoint of hours than almost any of my drives in PC usage. Since I do not leave my PC's on for 24 hours a day, and when working they were all off while I was gone, I would say that I typically put less than 1,000 hours a year on any of my PC drives. A drive in an NAS that runs for 24 hours a day at the end of a year will have accumulated about 8,700 hours. I would doubt that I would ever accumulate that many hours on a PC drive, because it would take about 8 years to get there, and by then, even if I'm still running the PC (unlikely), I would have upgraded the drives.
This is why, many people set their NAS units to spin the drives down after a period of inactivity.
Remember - the NAS is not a backup plan.
Backup = Good, No Backup = Bad