DxOMark published their results for the Nikon D4s camera. The surprise (at least for me) is that the Nikon Df still has a better overall low light performance (3279 vs. 3074 for the D4s). For now the Nikon Df is still the "Lord of Darkness".
Here are few more D4s comparisons with various Nikon and Canon cameras:
"Although Nikon has improved a number a features, and added several video-oriented functions most of these have been achievable by upgrading the processor. Although some slight gains have been made minimizing shadow noise and expanding the dynamic range, sensor performance is largely unchanged. That’s not to say the D4s isn’t compelling in anyway, as it is, but anyone that expected significant gains in performance are going to have to wait for the Nikon D5."