How much data in one volume do you have or want? I'm assuming 8TB. With 4TB drives RAIDs like RAID 10s may make less and less sense than striping a pair (RAID 0) of drives and using something like CCC to backup that pair with another pair either in a striped or concatenated (spanned) format to get a 8TB volume.
The advantage of concatenated is you don't loose everything when one drive fails, usually just the data stored there. It is slower than a RAID0 set, however, but does your use case need performance for data backup purposes?
With this strategy, should there be any OS anomaly that corrupts drives or erases files, you have a good chance that the backup set is unaffected. You will have some history of files to recover so when you accidentally erase a file and empty the trash, it will still be there on the backup set.
Just saying that perhaps buy your Thunderbay (the older Thunderbay IV is less money because it does not have the HW RAID built in) and install two drives. Use disk utility to set them up a striped RAID 0 , that gives you a 8TB volume. Copy your data from the guardian to the new RAID, that will give it a good infant mortality check. You could keep the old Guardian around, you can switch it to RAID0 or span (concatenated) that will give you a 8TB volume (the independent disk mode only works with fire wire). The advantage of using the guardian for your backup is that is an independent enclosure, giving you some protection against a Thunderbay box failure.
Just saying there are a couple ways to skin that cat and it the better way depends on if you are looking for redundancy and backup or redundancy for availability (less down time). If you set the Guardian to independent disk mode, you could actually use Disk utility to RAID 0 the two drives there and then mirror the Thunderbay... but I wouldn't want to do it that way unless you are looking for high availability
