Downscaled 4k on 1440p would look better than 4k on 1080p, because there are more physical pixels, so you'll see more detail than 1080p already, you'll land inbetween 4k and 1440p. Upscaling is different, because you're taking the same information you started with and making it bigger, allowing you to spot artifacts, etc, easier. Hopefully that was a simple explanation.