I have 2 27'' displays: 4K and FullHD native resolutions. If I set 1920*1080 on the 4K display, the image looks blurry in comparison with FullHD native display.
I would not be surprised with any other resolutions, because I understand that pixels are physical squares, so if a ratio between 2 numbers is not a whole number, we could see artifacts.
But in this particular case FullHD is exactly 2 times smaller than 4K on each dimention. So each dot in 1920*1080 image should take exactly 4 physical pixels on 4K display and I do not see any reason for blur.
So why the blurriness appears?