Can IPhone 11 pro max play 4K videos?

Hello ImperfectBeing12345,

Thanks for posting to the Apple Support Communities. I understand you want to watch videos in 4K on your iPhone 11 Pro Max. I'll be happy to explain.

If you're using iTunes paid content or rentals , this resource might help: Find and watch movies with 4K, HDR, Dolby Vision, or Dolby Atmos

As for the other apps, it would be best to reach out to the developer for further assistance. This link can help explain how to contact them: How to contact an app developer

Thanks and have a great day.

You need to select the quality setting within the app. Not every YouTube video offers a 4K option, you may only have 240p, 360p, 480p, 720p and 1080p etc... some videos were uploaded to YouTube in 1440p and 4K aswell. It depends on the video and the uploader, not on the phone. I guess it's the same for Amazon Video.