Curious as well. Couldn't we create a 4TB raid with 3 1TB 7200rpm drives (travelstars are $75/ea currently) and 1 1TB SSD with the SSD in bay 2,3 or 4? I think we'd see 600+ MB/s with that Hybrid configuration while staying within power restrictions. That combo shows much better savings vs 4 1TB SSD's.
What would be the point in your scenario? Any RAID is limited by the slowest drive. Thus, having 1 SSD with 3 Mechanical drives would be pointless. You might as well save $500 and just buy 4 Mechanical drives....
Remember, a RAID array will read from all drives simultaneously, thus if even one drive is slower than the rest then the entire array waits on that drive. In your scenario, the SSD will read it's "piece" faster than the rest, but won't be able to read its next "block" until the Mechanical drives complete their "pieces"....