DxOMark published their test results for the Nikon D4 camera. The D4 automatically took the #2 spot, based on overall test score and just 2 points after the PhaseOne IQ180 digital back. Another interesting aspect is that the Nikon D3s got a better low-light ISO score than the new D4.
DxOMark doesn't have any test data for the new Canon EOS 1Dx camera yet. I will post another comparison later between those two cameras, since the 1Dx is the only real D4 competitor.