I totally agree with the likelihood that the 1 Series use higher tier componentry. There must be some reason people claim that 1 Series RAW files can be pushed and pulled more than lower tier bodies.
I use the spot focus feature tons so that's a knock against the 1D4.
You're probably right that the 1D4 may retain more value long term given that it is the last in the line of APS-H bodies and is very popular among wildlife photogs. No doubt it is a great camera. Just not convinced yet that it would be a step up from the 5D3 -- perhaps more of a lateral move....I mean I realize the build quality and FPS are an advantage but as has been discussed it does lose out on the 5D3 for some features (5D3 has 2 more cross type points, spot focus, low light servo performance).
Yes, for some reason, 1D files can be manipulated more in post, at least in my limited experience. I also think Canon files can be messed with a bit more in DPP than Lightroom, for example. But, Lightroom wins in the interface department.
I think the files look different, too.
It is possible, given good exposures to start with, that the 5DIII file can be cropped to match a 1DIV file with no loss in IQ. It may be possible to do the same with a 1DX file- crop it to a 1.3x FOV with no loss in IQ. The "reach" arguments are going away as technology marches forward.
I have noticed that Canon seems to be splitting up feature sets across the product line. Note the stellar center point in the 6D vs other current FF offerings. But, Canon made the body smaller than the 5DIII, which isn't appealing to me.
If low light AF performance and spot focus without a Big White are important, then a 1DIV isn't an upgrade over a 5DIII.
There are 3rd party focus screens available for the 5DIII, but ease of use doesn't come to mind. If focus screen use is of importance, then the 1DIV becomes an upgrade over a 5DIII.
The 1DIV also lacks all of the cool stuff that can be done with current Canon RT flashes through the camera menu. Although it may be possible to address this with a firmware update, it hasn't happened yet.
Given current offerings, the 1DX is the only one that has limited occurances of the word "or" when describing feature comparisons between current models.
The one major thing that the 1DIV has over the 5DIII is value in the used market. If the 1DIV is in good to excellent shape, you can flip it with little effort, and get more funding towards a 1DX or try out a 6D.