I am sure that there will be a noticable difference between the upscaled picture and the native 4K one - provided the input in both cases is true 4K resolution and with detailed scenery, where it makes a difference. Otherwise why would people like Abrash even bother to mention the ultimate goal of 16K per eye?
However, it is true that currently there will be limited true 4K input (though due to 4K monitors being around, it is coming sooner or later). And the real issue is, that nobody will have a machine powerful enough to run native 4K content of a game offering the kind of detail with 80-90 Hz to make it a fun experience. Either you have a simple grafics game a la Fruit Ninja which you can power with your high end GPU on the full 4K resolution, but then you indeed hardly notice it, or you look at a nice Eliute Dangerous slide show.
In the first instance I fear it rather will be practical for watching movies in a virtual cinema with a really good resolution of the virtual screen in front of you.