we are playing with H265 in 4K. it works up to 2880x1800, which is the biggest non-4K screen we can test. On the 4K screen, if the hdmi is set to HD, the 4k video starts, but only the left-top quarter is visible, as expected. However when I set the hdmi output to 3840x2560, the output will be black.
Also, if we try to place 4x HD H264 images to fill a 4k monitor, the same thing happens.