It is hard to imagine one of Netflix's most successful rivals failing to have such basic functionality, but in early 2018, Hulu removed the option to stream content in 4K from its service altogether. But that's starting to change.
Now, 4K is back, but only for Hulu Originals programs (like The Handmaid's Tale, Harlots and Catch-22). "We hope this helps!" A Hulu support page says 4K Ultra HD content is now only available on Apple TV (5th generation and up) and Chromecast Ultra. And the service notes that "while Hulu's library of HD and 4K Ultra HD supported content continues to grow, at this time, it's limited to a select list of programming, like all Hulu Originals".
You're also going to need the right device to take advantage of the content Hulu is now offering in 4K. Hulu's Live TV guide works just like any other interactive guide you've ever used, showing you the broadcast schedule for channels that are included in your subscription.