If everything is the same (codec, bitrate, etc), 1080P will look better in anything but a completely static scene because of less blocking/artifacts. But that’

If everything is the same (codec, bitrate, etc), 1080P will look better in anything but a completely static scene because of less blocking/artifacts. But that’s an unrealistic comparison, because 4K often gets a better bitrate, more advanced codec, etc. If the 4K and 1080P source are both “good”, 4K will look better.