Hulu had a feature for a while with their streaming service that would enable users to watch movies and TV shows in 4K. The feature seemed as though it was only available for certain devices and only on Hulu Originals.

Confirmed in a tweet by Hulu that can be found below:

4K content from Hulu can be streamed from the Apple TV 4K and a Google Chromecast. However, this is not compatible with the iPhone, iPod Touch or iPad.