Why is 4k video not decoded by hardware?

Question

Video cards

Hello.
Central processor - AMD A10 5750M, respectively, with built-in video core AMD Radeon 8650G, monitor resolution - 1366*768.
At the same time, video in FHD works fine, the video player loads the CPU by 2% on average, and the video card by 3-5%, but when playing 4k video, it starts to be processed programmatically, loading the CPU completely, in both cases the H.264 codec which the video card can decode.
What kind of nonsense?
Hardware decoding does not work either on the bus or on the line, both on the free and on the proprietary driver.
The corresponding option is set in the player.

Answer 1

The same thing will happen when 4k is decoded by hardware, you will have 4k monitors, but you want to watch the video in 16k