One of the biggest drawbacks of Apple TV 4 is that you cannot watch videos while browsing your library or working in any other application.
For a smart HDMI streaming device, this function seems a must. For many, the reason for the absence of this function in Apple TV 4 is not clear, especially considering the fact that Apple, introduced a similar function in iOS 9 for iPad.
Now that Apple TV 4 is running on its own tvOS, it shouldn't be difficult to implement this functionality.
Developer stroughtonsmith modified one of his apps for Apple TV 4 and to his surprise it worked.
In the video, you can see that this function works great and it makes you wonder why Apple does not add it Apple TV 4.
The A8 chip inside Apple TV 4 is powerful enough to support picture-in-picture, so it's not clear why Apple doesn't add this feature to tvOS.
However, hopefully Apple will add picture-in-picture functionality in Apple TV 4 in one of the next tvOS updates.