How some android phone supports 4k video capture and playback while some pc doesn't.?
Most smartphones have dedicated video encode and decode hardware. It's specifically designed for that task, and it has a fast connection to the memory, and often the decoded video frames can be accessed directly by the compositor hardware so they don't have to be copied (or blitted) into the framebuffer. You wouldn't be surprised that a handheld digital video camera can do 4k video capture: it's specifically designed for that purpose. Your smartphone works the same way.

Most PCs don't have this special hardware, so they have to do video encode and decode with software, on the CPU, or with a special compute shader on the GPU. Either way, that's always going to be slower and less power-efficient than having special-purpose hardware.