Face detection/recognition function that will be working in the viewfinder (maybe some type of a hybrid viewfinder? Nikon had several related patents)
Improved video, I have no other details on that but my guess is 1080p/30/25/24 and 720p/60/30/25/24 similar to the Nikon D800
Uncompressed video out through the HDMI port
Ability to assign the two buttons on the front of the camera to smooth aperture control during video recording
Improved 51 AF points
AF detection range will go down to EV-2.0 (the D3s went to EV -1)
Autofocus system: 9 cross-type sensors that are operational up to f/8
The Nikon D4 is clearly made to compete with the Canon EOS 1Dx. I will stay conservative on the announcement date, but there is a very good chance that the Nikon D4 will be announced right before or during the PMA show in Las Vegas (January 10-13, 2012).