I've owned the 1d4 from the first batch here and had until I bought the 5d3 in march last year. The 1dX I've owned since august last year and to question the IQ difference is to me just to argue to argue to argue
No doubt that the 5d is better than the 1d4, with sharpness, I can give edge to the 1d4, but color and noise is way better on the 5d3. For me personally a fullframe picture is always preferred because I like the shallow dof and smooth, clean rendering. I demands more of a lens, but the best lenses shines much more on a FF camera. The AF system is also the biggest factor in getting a sharp image when you're shooting anything that can't be done with MF (and my guess is, if you buy a 1d4 or 5d3, you're not into MF) and there also, the 5d3 is way better, especially tracking subjects and when the light dims a bit where I found the 1d4 to be pretty poor.
Then there's the 1dx.. It's the only camera I have ever been completely satisfied with, it's truly remarkable and it IS better then the 5d3, better sharpness, more accurate color (less noise when using ColorChecker-correction) much less noise at even moderate iso values and better DR. I can work the files harder without them collapsing. That on top of everything else that's better with it than the two others is what makes such an amazing tool to use.