RAID is not Backup. The R is for Redundant; RAID protects you from a hardware failure, but not from things like accidental deletion.
A RAID volume can be a good target for a backup system such as Time Machine, but RAID is not itself a backup.
That said, you didn't include the RAID type you were considering in your post...
RAID 0 - increased performance bit increased risk (you actually have a higher chance of data loss)
RAID 1 - same performance as a single drive (maybe slightly lower performance), but you are protected from a single drive failure. Half of available space is lost to redundancy
RAID 5 - good balance between performance and protection, but you need either a dedicated enclosure or a dedicated RAID Controller. This is a very commonly used RAID Level in NAS devices. You can often expand a RAID 5 volume without loosing data, but it depends on the controller or NAS you are using.
Best guess from your post is that you have 4 1 TB drives and are considering RAID 5 in an external enclosure - maybe connected via FW800. That would be a good target system for backup software like Time Machine, or for a cloning strategy using software like Carbon Copy Cloner or Super Duper.
Can't really do much better than that based on the info in your post. Just remember that RAID by itself does not a backup make!