My guess it's using some tech similar to ZFS. Creating a 4GB log drive for writing, and the rest as a cache drive. Sounds like they abandoned the adaption of ZFS to make their own interpretation.
It doesn't sound like a cache at all. Think of it like their old "hotfiles" relocation to the faster part of the disk, only now they're relocating the blocks from entire files to the faster volume in the corestorage group.
I wanted ZFS as much as anybody, but NetApp did push the state of the art in some areas of filesystem design and the first thing everybody who used NetApp said when they heard about ZFS was "cool, it's like a NetApp". Sometimes it's not trolling; it's just enforcing your rightful patents.