Added the 4th disks but it is not RAID1

I sometimes think problems are inverse to the backup situation. That is if you have no backup you are more likely to have problems, but if you are current you won't have any problems.

Not really, but it certainly is less stressful if you get the dreaded dual HD failure and have a recent backup at hand. The most likely time for a drive failure is around the resync process as it stressed the drives more more than at any time, for they are undergoing a very high level of activity for hours on end. If you have a drive that is susceptible to a failure, just not yet weak to the point of failure in regular operation will be more likely to fail under that stress. If the resync is because you have just replaced one drive, then this would put you into the dual drive failure. While not common, it has happened. A separate current and complete backup is your protection in this situation.

Remember what I am talking about is not a problem with the controller, but the drives themselves. When we speak of drive failure, it is not an if, but a when. Hopefully, drive failure will not rear it's head until after many years of usage, but I have lost drives in their first month of service in an NAS and I have removed drives after years of 24/7 operation, not because of failure, but because I was replacing them with larger drives. Many times I use these drives to support backups of data.