Jay - do you want maximum storage or maximum redundancy? The more redundancy the less actual storage you have. For example:
Raid 0 is no redundancy, so you get all 4TB for storage but the pooch is screwed if something happens to the disks.
Raid 1 is disk mirroring, so you would halve your storage for the price of having an exact duplicate of the drives.
Raid 2 uses but striping, which is pretty good for maximizing your use of the drives, but can be very slow to recover and less robust that '3'
Raid 3 uses striping at the byte level plus additional check data. If you can afford the use of space, I would recommend going with this one.
Raid 4 really isn't used.
Raid 5 is the most robust and easiest to recover from in the event of a disk failure.
All these techniques work fine with 4 disks, but you really only get 1 disk worth of storage space except with Raid 1 where you get half the storage of the total array.
If this seems too techie, you may find more useful information at
http://en.wikipedia.org/wiki/Standard_RAID_levels
The type of raid you want affects the systems that are applicable.
I have a friend who runs a small ISP who I can ask raid questions to, so I'll ping him for his recommendations.