So far the 1D4 is ever so slightly sharper which I put down to more consistant and accurate autofocus.
Now that I have my hands on it and as I used to have a 7D it makes the '7D vs 1D4' threads look a bit idiotic.
Love the extra focal points,
Colour, noise and overall IQ are the almost same when shooting Raw, the 5D2 having less noise at ISO3200 onwards.
The 1D4 is slightly lighter (my 5D2 is gripped) and the only thing that really matters to me is the better autofocus and AF points.
I would be losing full frame and 6mp in the swap so am considering a 1D3 to compliment the 2 5D2's I have instead.
One thing I have to add, the way the image comes across is like the 40D, odd I know but it's almost as if they took the 5D2's sensor, and cut the thing down in size and in doing so lost 6mp. The reason I mention the 40D is that out of all the SLR's I've used (40D, 50D, 7D, 350D, 5D, 5D2, 1D4) Only the 40D, 5D2 and 1D4 have the same sort of 'look' to them.