I'm sorry, but I don't even know why you are asking the question. Video resolution is whatever you set, up to the maximum information in the video file. So for 4k, if the image is 3840 pixels wide and you display it at "native" or "original" or whatever the video software calls it, it should use 3840 pixels as the width. But if you shrink or stretch the video window, the player will try to adjust the image, either stretching the video over more pixels on the screen or dropping some to fit into a smaller window. The file will still be a 4k video, presumably with the same 3840 pixels of information.
So what is it you are trying to do? You say you play 4k videos in quicktime and from your description it's doing what you say you want, which is to have one pixel of information be one pixel on the screen. Or do I misunderstand what you mean when you said, "that's exactly what happens?" I took that to mean that quicktime opened a window roughly 80% of the screen width to display the video. But if what you meant was that it went to full screen, then change the setting in Quicktime to use native resolution.
