Apple TV to 4k Apple TV

For content in the iTunes Store that is available in 4K, you should be able to access that 4K version if you bought the HD version or better. The 4K access is not available for SD purchases. The 4K access is not available for HD purchases from a reciprocal service. Downloaded copies of movies from the iTunes Store will be in HD, not 4K. 4K is streaming-only.

Basically, in the iTunes Store, the HD and 4K version are the same item for purchase, and you don’t have to re-purchase titles (you can’t even).

About 4K, HDR, and Dolby Vision on your Apple TV 4K - Apple Support

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

If I buy an HD movie on Apple TV/iTunes will it be upgraded to 4K? - Apple Support Community

Welcome to the Apple Community. 

They'll be upgraded

Yes, I recall being rather surprised at the time I got mine, especially after there being a big difference between SD and HD prices.