Well your ordinairy 4kTV already has one in the first place

Indeed :P Hence why I believe House is referring to native 4k... dedicated upscalers almost always do a much better job though, quite expensive though.

Why this insane focus from MS and Sony on 4K??

Wish I knew. I would say dollars but this could be a big risk to both. Most people don't even own a 4k TV....

Ignoring variables we don't yet know, the Neo having 3x times the GPU power of a PS4 (so about 5.5tf) should be enough for it to compete with the Scorpio in the 4k space. They could just run the games at a slightly lower resolution, e.g. 2060p instead of 2160p. I doubt even a pixel whore like me would notice that much (though i'll have a check in a minute).

As it currently stands though, i don't expect either the Neo or Scorpio to be hitting 4k as a primary standard. At least not without the PS4/X1's current graphical standards taking a dip.

Edit: I just tried out 2160p vs 2060p in MC. The difference is slightly noticeable, but it's small enough that compression makes the difference pretty much disappear: http://i.imgur.com/NlAMJ80.jpg (so that people's screens aren't overtaken by a giant image, i just look slices of both images, but the slices are native).

Haven't you seen the 2017 line up of MS 4K tvs? :p From Sony I can sort of understand it being invested in 4K tv, 4K blu-ray and 4K streaming. Yet Sony prefers to stay quiet about it and go ahead with the psvr launch first.
From MS it seems more like a disruptive tactic. Focus attention on the 4K buzzword away from the imminent VR release. And downplay NEO, say they're coming with a real power upgrade in 18 months (which might also support VR) It works, psvr seems largely forgotten. 4k 4k, forget VR.

Developers balancing two(or four...) versions of the same game will be difficult. Do they devote extra man-power, time and money into beefing up the 4k version? Or do they compromise and the differences between the 4k version and standard version will be minute...

Again (I wonder if I'll ever get tired of repeating this), there's general misconception that 4x pixels needs 4x GPU power...RL benchmarks put this at around 3x or less.

Here are 2 cards with 5.6 TFLOPS from nVidia and AMD and how they scale with resolution:

http://www.anandtech.com/bench/product/1715
http://www.anandtech.com/bench/product/1721

and here are 2 with around 8.5TFLOPS (you can choose any card really):

http://www.anandtech.com/bench/product/1714
http://www.anandtech.com/bench/product/1720

That said, NEO is rumored to be to be 4.19TFLOPs, which is ~2.3x PS4, so not quite there, but now that MS revealed some of Scorpio's specs, maybe Sony will bump that up a bit.

Why this fascination 4K all of a sudden?

Make your games run 1080p 60fps and make them look photorealistic.

Scorpio is about 4x as powerful as Xbox One, so imagine how much better their games would look if they didn't focus on 4K.

If you are just pushing more pixels, the CPU doesn't have much to do with it. If they keep a similar CPU and upgrade the GPU, 4K @ 30 will be more feasible than 1080p @ 60.