OWC is having a special on the Thunderbay IV with additional savings when drives are purchased:
We are now offering savings of $20 off two drives or another $40 off with 4 drives ($60 total) purchased with an OWC Thunderbay IV 0GB 4-Bay Thunderbolt Enclosure Kit which offers up to 895MB/s sustained data rates with 4 x 7200RPM HDDs (now $479).
Qualifying drives include select Toshiba, HGST, and OWC SSDs. Free delivery within the 48 contiguous US States.
See the MPG review of the Thunderbay IV.