Checking Storage Spaces use-case

Hello,

I'm interested in using storage spaces but I'd like to check the validity of my usage case before purchasing additional hardware.

(In this usage case all the references drives are blank; the data is presently being stored on Amazon S3, temporarily)

I have three, 2TB hard drives. I'm looking to purchase two 4TB hard drives and an additional 2TB hard drive. I'd like to use the two new 4TB drives as my primary storage space, and the four 2TB drives as my 'backup' to the 2x4TB pool that I'll be using. Is this possible? How should I provision my storage space?

Thank you :)