That is old technology. It is irrelevant today because scaling the display is a part of all modern displays. Scaling and letterboxing are not going to give you a full-screen display, but just backing up about 2 feet will make those pixels you are worried about blend into the display. It would be more important to me to see what an HD render would look like on a 4K display at a normal viewing distance than to have the preview scaled down to only show the actual pixels. The 4K display probably has quite a few more dots on the screen than there are pixels displayed, anyway. If you are rendering HD and watching on 4K, and the viewing distance is about the same as the screen's diagonal, most people will see a softer image than if you were rendering 4K. Change the viewing distance to twice the screen's diagonal, and maybe two out of a hundred people could tell the difference between an HD clip played on a 4K screen, and a 4K clip played on a 4K screen. Increase the viewing distance to three times the diagonal, and there is no discernable difference between 4K played on a 4K monitor and HD projected on an HD monitor because human eyeballs can't resolve any higher than that.
I have a retina-display Mac, and there are way more pixels on the screen than I can see unless I use a magnifying glass. The display has about a thousand more pixels than the highest resolution you can set with the display controls. Two thousand fifty-six pixels wide is the highest resolution you can set. Three thousand four-hundred fifty-six is the actual pixel count of the Retina display. Those extra pixels on the screen give you better antialiasing and provide a higher contrast range.
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