This, and all consumer 4K UHDTVs are fully backward compatible with 1080P HDTV which is the maximum output that standard DirecTV boxes output. To receive 4K, you will have to have their newest Genie receiver, the HR54. Depending on your 4K TV, you MAY need a C61K, and, of course you must purchase their 4K programming.
https://www.directv.com/technology/4k
Other options include streaming 4K from the Internet via a buitl-in app. You can also use stand-alone boxes like Amazon FireTV & Roku. They essentially do the same thing as the built-in apps, but are faster and have slicker GUIs.