Look at the edition, not the poster
A sharp poster and a high movie rating say nothing about the resolution offered by a viewing service. Open the edition and check its quality information. 1080p usually describes full HD; consumer 4K commonly describes 3840-by-2160 pixels. A destination may reserve a particular format for certain devices or plans.
More pixels are only part of the picture
Compression, the source master and the display all affect what you see. Dark scenes can expose compression that is less obvious in daylight footage. Older material may retain grain that belongs to the original image. Neither grain nor black bars automatically means the stream is broken.
When playback keeps stopping
Try the player's automatic-quality option or a lower resolution and observe whether it becomes stable. If the problem continues at several quality levels, check the connection or the service's help. Avoid making simultaneous changes to the router, television and browser, because then you will not know which adjustment mattered.
For a large screen, confirm both audio and picture before the film starts. HDR capability, resolution and sound formats are separate questions. The device guide helps you distinguish browser access from a destination app's playback support.