How does this work? Will it just upscale or is e.g. text still rendered at 4K? Rendering not at native resolution results always in blurred edges in my experience.
Here's an album with a pair of screenshots from my own 4k display: https://imgur.com/a/7AHZZZv -- the scaled one is how I normally use it.
The Windows UI scaling slider behaves in exactly the same way, though fewer apps include 2x or 3x bitmap resources.
This makes it sound as if macOS upscales a 4K render when displaying to (for example) 5K monitors, but on a 5K monitor everything is ultimately rendered at a full physical resolution of 5120x2880. But in the Displays Preference Pane, the logical resolution is set by default to 2560x1440 (2:1). One can choose a logical resolution of 5120x2880 (1:1), but I can't imagine anyone working like that.
Whatever-sized display viewports are just crop windows into it, and the crop window in internal canvas terms is scaled as necessary, and then it renders the vector canvas onto the raster viewport.
