Here is a $200.00 and $420.00 angle.
1. Adding an external 4 drive RAID is not going to get you to 8TB (if all HD's are 1TB) or 1400+ speeds without a card to connect the internal to it. This is obvious. With the R644 I have 8TB with the internal/external combo
2. Since your internal RAID is "fast" enough for your "current" needs, buy 2 bare 4TB HD's
a. Dump the internal RAID files on one 4TB HD. Place this 4TB drive in the optical bay
b. Copy that 4TB HD. This will be your Backup. Place it in your vault!
c. Remove all "non active" files from the internal RAID. Use Drive Genius 3 or above to defrag.
d. Internal RAID will be faster, larger and have that new car smell!
3. Remember to save "current" files in both locations or backup to that 4TB HD often
4. Keep "active" and "archive" separate since space is an issue.
Plan (B) Dump the internal RAID to one 4TB HD ($100.00), Place (4) 3TB HD's = 12TB ($320.00) in the internal and rebuild the RAID. Dump the files to the new 12TB RAID. Try and fill that up before summer!