Upgrading Itunes purchases from HD to 4k?

Hello K_catarino,

Welcome to Apple Support Communities!

We want to help you get the most from your content.

You should be seeing all available options to play the movies back, depending on the content.

Try here:

"1. Open iTunes Movies or TV Shows on Apple TV.

2. In the menu bar, navigate to any of the following categories:

• Purchased: See the movies you’ve purchased on the iTunes Store, including purchases made on other iOS or iPadOS devices and purchases by Family Sharing members. 
• Top Movies or Top TV Shows: Browse the top items in the iTunes Store. 
• Wish List/Favorites: Find items you’ve added to your Wish List (Movies) or Favorites (TV shows) but haven’t yet purchased or rented. 
• Genres: Browse by genre. 
• Recommended: Find recommendations based on your previous purchases. 
• Search: Enter a search term to find movies or TV shows. 

3. Select an item to see ratings, descriptions, and purchase or rental information.

4. Do any of the following:

• Watch a free preview: Select Preview.
• Buy an item: Select Buy, then confirm that you want to buy it. For TV shows, you can buy an entire season or individual episodes.
• Rent a movie: In some regions, you can rent movies. Select Rent, then confirm that you want to rent it.
  • You have 30 days to begin watching a rented movie. After you start watching it, you can play it as many times as you want within 48 hours. During the rental period, you can download the rented movie on one device at any given time and also stream it on another. For example, you can start watching a movie downloaded on your iPhone, then finish watching it later on your Apple TV. Once your time is up, the movie is no longer available.
  • Add an item to your wish list: When you find something you might want to buy or rent later, select Wish List . To view your Wish List, navigate to Wish List in the menu bar.
  • Remove an item from your wish list: Navigate to Wish List in the menu bar, then select the item. Select Remove  on the item’s details page.

When you buy or rent a movie from the iTunes Store, it automatically plays in the highest-quality format available for your Apple TV, including high-definition (HD) and 4K resolution formats. These videos might also feature high dynamic range (HDR) in HDR10, HDR10+, or Dolby Vision, and Dolby Atmos. For more information, see the Apple Support article Find movies with 4K, HDR, Dolby Vision, or Dolby Atmos in the Apple TV app."

This Apple Support page has the full details: Watch iTunes movies and TV shows on Apple TV - Apple Support

We hope this helps understand your best options for playing back your movies.

Cheers!