Single 8 disks will give you more flexible way to deal with space, and disks, while using 2 of 4 disks will let you maybe more organized with your data, as if you want to add extra space to a the NAS or iSCSI users you have.

The easiest way now a day to save a backup for all the system is to have an external HDD and save a backup on it, that way you will be in double safe.