Convert movies to 4K

When you get a 4K TV, and have an Apple TV 4K box,

or when you get a 4K TV that has the Apple TV app,

then you get to have access to the iTunes Store 4K movies automatically, when available.

If you previously bought an HD movie from the iTunes Store, you might have access to the 4K enhancements on a compatible device.

Find and watch movies with 4K, HDR, Dolby Vision, or Dolby Atmos - Apple Support

You don't, an Apple TV is not a media conversion tool it is a Internet streaming device. Not all content available on it is in 4K, however there is more and more becoming increasingly available. For example, if you subscribe to Netflix 4K content you can use Netflix 4K CONTENT to see what they offer.