I'm still surprised they picked the D4 as the better high ISO ... :S
Having used both, my humble opinion would be the the D4 might a touch cleaner (1/3 of a stop or 1/2 stop at most) all the way to ISO 6400. Then from ISO 6400 onward, it seems the 1DX is better. For the first time I shot some ISO 16,000 this week-end with the 1DX and they were actually very usable!!!
1DX, 24mm f1.4L II, 50mm f1.2L, 85mm f1.2L II, 135mm f2L, 24-70mm f2.8L II, 70-200mm f2.8L IS II : D800, D4, 24 1.4G, 35 1.4G, 50 1.4G, 85 1.4G, 24-70 2.8, 70-200 2.8 VR II