If one drive dies (or even coughs badly) you'll loose everything on the array. There'd be not harm to the actual drives, but recreating 8TB of data could be annoying.
Ah okay that's good to know... It would definitely be annoying... I'm assuming that if I put them into RAID 1, if one drive dies, I'll be able to just swap it out for a new one and everything will go back to normal? Also how much space will I have to play with with 4x2TB drives in RAID 1?