I have the Drobo 4 myself. I love it. If I were to have your situation I would choose 2 of the old 4 units as the more you can divide your hardware the less the possibility for a single catastrophic failure to wipe you completely out.

If one DROBO fails, you still have 1/2 your files still if it effects the drives plugged into it.

Just always looking at the safer side of backup.

In my unit I have 2 1.5TB drives and one 500Gig drive and I have every file I have ever created on it and still have room and one empty bay.

That's over 35,000 RAW files and multiple processed versions of many of them.

Roman