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:
We have good news! 4K streaming was just re-released today on select devices: https://t.co/5BjUFgexXk. You’ll be able to watch any of our Hulu Originals in 4K. We hope this helps! 👍
— Hulu Support (@hulu_support) July 9, 2019
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.