Resolution describes the played image, not the catalog artwork. Opening a large poster on MoviesJoys does not establish that the film has a 4K edition. Check the available quality on the viewing offer and the player you select.
Match the setting to the screen
HD or 1080p can be a sensible choice for everyday viewing. A supported 4K edition may reveal more detail on a larger screen, but the display, account, connection and device all matter. Higher resolution is useful only when the rest of the setup can deliver it.
Remember the source
Compression and the underlying transfer affect the image too. Restorations can look different from older editions of the same film. Compare edition information when it is available rather than treating a single resolution number as a complete quality review.
If the picture keeps changing
Adaptive playback can reduce or increase quality with connection conditions. If the player permits manual selection, test a lower setting to see whether interruptions stop. That test tells you more than repeatedly reloading the same stalled scene.