What do you have that's taking up 4TB of data?
Upon reflection, there's really only about 2.5 TB that would require backup, the rest is on-site backup of the original data and I obviously wouldn't be backing up my backups.
A great deal of my data is audio (my music collection, every single episode of NPR's Car Talk since February 2002, more than 3,000 audio books, plus photographs (mine and SWMBO's), games, income taxes, Rosetta Stone, etc. I have 6.5 TB of capacity, of which (including backups) about 5 TB is being used.
No doubt I could go through and get rid of at least one TB of data I'll never need nor look at again, but with hard drive storage so inexpensive ($80 for 2TB at NewEgg) why bother?
It looks like my best solution is to get a couple more 2 TB drives (WD20EARS, probably) to use in my external dock as backup drives and keep them at my neighbor's house.
tanstaafl.