I had a mkIV and now a 1Dx. The x is a significant step forward but I really must say that the IV does not disappoint either and you may well prefer the crop body.
I bought 1DX last week finally, it is an incredible camera, couldn't be happier.
Wish if Canon didn't discontinue 1D IV, it was a great camera, hope they can produce another one say 1D V maybe with crop 1.3x factor and maybe 15fps and 20mp or even 18mp and high ISO can go up to 204xx0 and giving nice usable ISO up to 51xx0.