Youtube and 4K playback

When playing youtube 4K videos, and not using the HTML5 option, I think it uses the flash player, is that right?

Do you know why, if you select "Original", what should be playing at 4K resolution, if you press "show info" it currently says "2048x1536" instead?

Is it because of flash or because of a change made at youtube to disable 4K playback (or because I'm not running with a 4K capable monitor/graphics card)?

(I'm using Firefox 15.0.1)