Choose the edition before the resolution
A movie can have different viewing editions across services and countries. Open the watch choices, then check the quality listed by the selected service. A poster is not a confirmation that every resolution is available.
A steady picture is worth keeping
Automatic quality can adjust to changing connection conditions in a supported player. If a stream keeps stopping, reduce the setting and pause large downloads before trying a higher resolution again.
What 4K needs
The film edition, service plan, application and display all need to support the selected quality. A television with a 4K panel is only one part of that combination. Check the supported setup on the service you actually watch.
Picture controls are in the player
Playback speed, full screen, Picture-in-Picture and subtitle styling depend on that player. They are not controls on the 1HD movie description page. Look for them after the video opens.