I think most folks with a 4-bay NAS use RAID5. This spreads the data across all the drives so that if any one fails the data can be retrieved from the other drives. If a second drive fails before you've replaced the first one you'll start to lose data.

My 6-drive Readynas allows for two drives to fail and still be able to retrieve the data.

However, as pointed out, if my house burns down then my data are toast (pun intended) unless I've backed it up offsite.

Sooloos, Roon, G series (with LPS upgrade), 800 series (with LPS upgrade), HD621, AC200, Prime and PS, DSPs old and new.

Purdeys, Moke, Leica, 'Blad. Only the best.