Buffalo provides Data Recovery services. Read about it here.
Even if you do this, it will still create a 1TB array. RAID 1 is a mirror, you can't create a mirror with two different sized drives. They both have to be 4TB in order to create a 4TB mirror. That's the way RAID works.
Thanks for the updates, all. Is there a way I can just pull one drive, delete the array, reformat the remaining disk, then rebuild based on the one I pulled out? I don't have a spare USB drive to feed in as a backup at this point.