The big differences that have affected how I shoot (bird) when compared to the 5D4:
- spot metering linked to AF point. This is huge. In fact, this is the one feature that I feel has substantially improved my keeper rate w/ the 1DX2 over any previous Canon body. It's a travesty that Canon only reserves this for the 1-series. I also shoot w/ the EOS R, and despite the WYSIWYG advantage of the EVF, the shots from the 1DX2 come out metered exactly how I'd do it in the R.
- 14 fps is the other major thing that has positively impacted my keeper rate, as it makes capturing the perfect pose on a flitting, jittery bird a ton more easy.
- improved AF. The differences are subtle, but definitely noticeable. The 1 feels a little more immediate and rock solid when locked on. I don't get as many missed focus frames in a burst as I used to.
- high ISO advantage: ~.75 to 1 stop of noise. Not as big of a deal as the previous items, but I'll take every stop I can get
One of the first things I did when I got the 1DX2 was to compare side-by-side shots in order to see if the MP difference was noticeable for bird fine feather detail. In the end, it wasn't all that big of a difference, and like you said, both bodies produce crop-able images.
The 5D4 is a stellar camera. If I wasn't primarily shooting birds, without a doubt it's the body I'd still own. Emphasis added because unless you're in need of more FPS and slightly improved AF, I feel that the 5D4 is the better camera.
I think that this is getting a bit out of control about what I stated: I did mean that I didn't see a noticeable difference between the images of the two bodies in MOST of my shooting situations. There have been many comments about the frame rate and quick acquisition of the 1DXII-I agree. This is a huge difference between the two bodies.
Another big difference is the incorporated wireless and "touch-screen-to-capture-above-the-head-shots" of the 5DIV. They are very different bodies, with different strengths, I was only trying to submit the notion that either would be my last camera on a desert island. With wi-fi. And power. And Profoto strobes. And the models from the last 5 issues of SI Swimsuit....I guess I'll just top there and hope that my point was clarified