I spent quite a while making a similar decision, although mine was not between the 1 DX and 1D4, it was whether the 1 Dx would be an improvement over my 1 Ds3 and 7D combo.
As the majority of my photography is wildlife, I've been using the 7D a lot to take advantage of the crop factor, I find lugging around anything more than a 300 2.8 is a pain especially when travelling abroad, however the relatively poor ISO performance of the 7D has been a drawback.
I prefer the AF system in the 7D over the 1 D4 as well, the 1 D4 's system was I gather finalised before the 7d's and particularly the spot AF selection is available with all lenses on the 7D whereas it's only on the 1 D4 with super tellies.
So the big advantage for me is the ISO performance of the 1 Dx over both my other bodies, along with a higher frame rate than the 1 Ds3, although I'll still keep the 7D as a backup and for situations on good light where I need the extra reach and don't have to up the ISO.
However I'll probably still get the next 7D mk 2 or whatever it is called as a backup as I'd always prefer to have a second body with me when travelling.
Will know if it's the correct decision when my 1 Dx arrives, hopefully later in the week, although Andy's new arrival will no doubt grab the headlines...!!!
