Current Nikon Live View (D4 and D800e) in low light is pretty much worthless to me. Way too much noise is not being buffered out. Where as if you are attempting to focus on the moon at night, it's great. Unless Nikon added a totally new AF engine to the D4s, I doubt it will handle low light AF any better than the D800 or D4, which is pretty hit and miss. I only mention Live view here, as I feel that a future design may find a way to incorporate the Live view to assist with AF even more. Point is fact is how well the Live View that the IQ250 has at night, i.e. total darkness.
One of the more impressive feats of the IQ250 CMOS is that it can be used in total darkness and the live view basically does become a night vision device. Not how this is done, but the Alpa review of it was pretty impressive. You can also assume that if Sony ever took this type of design to a 35mm camera the AF may be able to focus in total dark as the Live seems to be able to display the image as in daylight. (at least on the IQ250).
As for the iso 400K, well, I will wait to see just how it looks, I am not expecting too much.
Paul C.