I assume you mean on a 1080p monitor?
If so, 4K “downscaled” to 1080p has 2 benefits: bitrate and chroka.
4K streaming has a higher data rate than 1080p (thru the same platform), and that data allows for less compression (grouping nearby pixels into having the same color and/or brightness). Here is [Apple Trailers vs YouTube](https://imgur.com/gallery/9nCFU) for some movie trailers (and keep in mind uploading these screenshots also goes thru compression.
Chroma is how much color.light info is stored. The best is 4:4:4, but most all platforms use 4:2:2 or 4:2:0 (many cameras don’t even shoot in 4:4:4). By playing a 4K video on a 1080p monitor, it is a 1:4 scale, so each 2×2 grid of original pixels now makes 1 new pixel, allowing for 4:4:4, giving even better color/light precision.
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