4TB+ External HD?? That is called Digital Suicide! I wouldn't do that with any data I rely on.
Get a NAS with built-in RAID. Install 4x2TB HDD in RAID 5. You end up with 6TB - which is equiv of 4TB+, but you don't live to regret when all your data is lost because the disk head seized!
Seagate 6GB/s 2TB 3.5" 7200rpm with 64MB Cache costs about 14,000 (+VAT).
Here is a link to someone you can contact for a NAS:
http://lists.my.co.ke/pipermail/skunkworks/2010-May/023920.html
However, much as I haven't seen a 4TB HDD myself, if you've seen them and want to commit the suicide still, please go right ahead:)