Just starting to test 4K video from my Carbon Cub. I am posting the tests on YouTube. In order to view 4K video you need a 4K monitor and a very fast internet connection. If you meet those requirements you need to click on settings in the YouTube video then select "4K" resolution.
If you have a MacBook Pro with a Retina display you can view the video around 2K. Shooting very high resolution video on the Carbon Cub is challenging because of vibration induced by the engine. I am still working through those challenges. Here is my first test.
If you don't see the "HD" next to the "gear" icon on on the YouTube display you are not viewing the video in HD. YouTube will test your internet speed and render the video to match your internet speed. So you may need to set a higher resolution and let the video load. 4K files are very big so it could take a while.
https://youtu.be/BNxlHWa6wXQ