Of course I would like more sensitivity and higher usable ISO levels and higher dynamic range. Given the incremental changes from the D3S I wouldn't expect that to happen in a significant way.
However, one thing that I think would be very useful (for any body, not just the D4S) would be a two-button reset that restores USER settings, not factory settings. There should also be a menu item to save/restore settings to/from flash memory, not just to a card (which gets formatted occasionally, losing the settings). In addition, a menu item that gets the camera to restore the user settings at power up (or you could leave that off if you really want). Lets make the camera fool-resistant.