Disney+ has launched in preview in Netherlands with movies such as Avengers: Infinity War, Star Wars: The Last Jedi, and Black Panther available in 4K Dolby Vision and Atmos. Here is an updated list.
Update: Captain Marvel in 4K Dolby Vision + Atmos confirmed.
Disney+ 4K streaming
Disney+ will not be charging extra for content in the highest streaming quality, specifically 4K Dolby Vision HDR and object-based Dolby Atmos sound.
The following 4K titles are already available in the Disney+ preview. We will keep the list updated at least until the official launch - possibly longer.

4K Dolby Vision (or HDR10) plus Dolby Atmos:
Avengers: Age of Ultron
Avengers: Infinity War
Captain Marvel
Star Wars: The Last Jedi
Rogue One: A Star Wars Story
Black Panther
Free Solo (in HDR10 and without Atmos)
Aladdin (1992)
Captain America: The Winter Soldier
Captain America: Civil War
Captain America: The First Avenger
Ralph Breaks the Internet
Mary Poppins Returns
A Wrinkle in Time
Christopher Robin
Princess Diaries (without Atmos)
Princess Diaries 2: Royal Engagement (without Atmos)
Nutcracker and the Four Realms

In recent months, the company has released multiple Marvel and Pixar titles on UHD Blu-ray so these are likely to also appear on Disney+ later.
Disney+ will launch in the US, Canada and Holland on November 12 and in Australia and New Zealand one week later. The company has yet to confirm launch details for other regions.