Piracy: ꜱᴀɪʟ ᴛʜᴇ ʜɪɢʜ ꜱᴇᴀꜱ

Yes, which is exactly what I'm stating. Showing a forcibly non-upscaled video (or one where you've manually tweaked the upscaling for that matter) is likely not what you want because there are no circumstances where that is what you'd watch on that particular screen. It could perhaps work as an example of how that video would look if you had a 1080p monitor of the same size instead of the 4k one you have, since it scales in a linear fashion, a pixel of 1080p is 4 pixels in a square on a 4k screen. But that's likely not what you want to test. Instead the thing you do want to test is "does it matter if I download X content in 1080p or 4k? How big is the difference really?" And if that is the question you need to let it upscale.