Start with the edition
A title's catalog entry identifies the production, not every video transfer available for it. Open the chosen viewing option and check the listed quality there. A high-resolution poster is not proof of a 4K stream. For older movies, restoration quality and the transfer can matter as much as the resolution number.
1080p on a smaller screen
A stable, well-prepared Full HD edition can be comfortable on a laptop or tablet. If a player offers automatic quality, it may adjust the image as connection conditions change. Check the active setting if the picture remains softer than expected. Repeatedly restarting the stream can make it harder to tell whether the connection has stabilized.
4K on a television
Check the service's supported app, plan and device requirements for that edition. Resolution and high dynamic range are separate properties. Do not assume an older adapter, browser or display chain supports every option named by the service. Test the intended screen before a shared viewing rather than relying on how the page looked on your phone.
Choose a sensible compromise
A slightly lower but stable resolution may be preferable to constant interruptions. Make changes in the player responsible for the video. If the picture is acceptable but dialogue is difficult to hear, inspect the audio track instead of adjusting unrelated picture settings. The sound and subtitle guide covers that part of the experience.