- the same 16MP sensor
- higher fps rate
- improved AF
- better low light performance
- improved video
- XQD and CF memory cards
I also expect the camera to have the new EXPEED 4 processing engine.
This is how Nikon described the D4s in their press release:
- enhanced image quality
- new image-processing engine
- more advanced autofocusing performance
The official Nikon D4s announcement will take place in February (probably for the CP+ show or shortly before), shipment will start in March.