I think that the RAW file size might be too big for the Epson P4000
Also not that the Leaf Aptus can be bought with a 30GB magazine. I suspect that this makes the P4000 less useful. Perhaps with the magazine and the large LCD on the Leaf you can review and edit images adequately.
In the RG forum James Russell described his battles with Leaf to get efficient Jpeg generation, but that may only be in tethered shooting. I don't know if the back generates JPG's as it goes (would be nice tho).
None of the medium format backs on the market allow for separate raw and jpg files upon download from the camera. Hasselblad hinted at it back in late 2005, but we haven't seen that feature added to their lineup yet.
When shooting with the Aptus 75, there is no jpg created that can be downloaded as a separate file from the raw capture. However, the preview that is created in-camera for you to view on the LCD of the Aptus, does become available as a not too great looking, but extremely quick jpg export from Leaf Capture.
JPG's can be exported with better quality, but it takes a bit longer. I believe that was the gist of JR's issue. I don't believe the Epson 4000 is even compatible with the .mos file, regardless of file size.
The Epson 4000 is a nice unit, it's too bad it won't take the .mos file. The 30GB magazine does offer sufficient capacity, although the Leaf Aptus screen is not as nice as the Epson 4000. But in a pinch...