I have owned the 1DMK IV since it's release, I also currently own 2 x 1Dx Bodies, both the 1DMK IV & 1Dx are amazingly good Cameras, as long as the Price Point isn't a factor, there's very little between them.
The 1DMK IV is older Tech, the 1.3 Crop Factor I like, which is the Main reason why I still keep the 1DMK IV, for Wildlife it's "reach" is at times very useful, I also do Underwater Photography & I use this Body for my Macro Imaging, effectively get's me closer than say the 5DMK III. The body is lighter & smaller than the 1Dx, Files from this Body are very good.
The issue with the 200f/2 is in the synchronisation between the older Big Whites & the 1Dx/5DMK III, in particular the 200f/2, you get a clicking sound in the AF motor on the Lens, when I went to the 1Dx I needed to send into Canon my 200f/2, 300f/2.8 II, 400f/2.8 II, and have new firmware uploaded to the Lenses, this wasn't required with the newer 200-400f/4 & 600f/4 II.
With the 1Dx you are getting up to date Tech, a huge improvement in the AF, 61 Point AF system, if your shooting fast moving subjects this is a huge boon to your Imaging, the Body is heavier & slightly larger than the IV, but until I go back to the IV I don't really notice it.
The 1Dx is compatible with the 600EX RT & ST E3 RT, this system is much better & so much more usable than the 580EX system, the new Flash system is not fully compatible with any Canon Cameras except the 1Dx & 5DMK III.
I do like the 1DMK IV, and if price is an issue I wouldn't hesitate in recommending this Body, but if price isn't an issue, I'de recommend the 1Dx, and in a Perfect world a 2nd Hand 1DMK IV as back up & something that gives you a little extra reach on your Large Whites.
If you haven't owned either, both will be great Cameras for you, having owned both, there's a real difference going from the 1DMK IV to the 1Dx, and it's all Positive.