why not cache?
"and is evidently not a cache - or at least not the front-end cache as the 32MB of RAM does that job."
Gee what a bizarre thing to note. Of course it could be be a cache. One little clue is that 4GB of flash is an awful lot bigger than 32MB of RAM so it can hold over 100x the amount of data. Secondly the flash memory is non-volatile so it can cache the frequently accessed blocks without having to wait until the data has been staged down from the HDD. It could also be used for a fast write cache (as it's NV), although that depends how it is implemented, how clever the controller is and what type of Flash is used. Of course it could be used for a separate segmented high-performance area, but 4GB is hardly enough to hold a full systems image and application executables.
The RAM cache in an HDD is there largely as a track buffer for sequential reads. It rapidly runs out of utility when the data set that is being read is moderately large. A hybrid makes sense if the flash is large enough, fast enough (especially for writes) and the caching algorithm is clever enough.