原留言 Hey, you misunderstood what "Viewport" and "Current/Optimal Res" means in the "Stats for Nerds" overlay on YouTube. You are, in fact, getting 4K in your demonstration - you can see that in the "Current Res" line. Viewport is merely the "virtual" resolution, which is only used for UI element scaling. Think of it like a retina display in a Mac: The operating system acts like an HD display was connected and scales the UI elements accordingly, but it's actually pushing all ~8 million pixels that the 4K screen can display. The reason for this is that otherwise all the UI and text would be really really tiny on high-resolution displays.
tl;dr: Ignore the "viewport" line and only look at "current res". It's not a bug, it's just confusing if you don't know about pixel density. :)