A high resolution can reveal more detail, but it is only part of the picture. The source, compression, display and connection all affect what you see. Start with the edition offered for the title and country you chose.
HD and 1080p
A stable HD stream can suit a smaller display well. A supported 1080p edition may show more fine detail, especially on a larger screen. If an adaptive player begins softly, allow it a moment to settle before changing every setting.
Selected 4K editions
Check the viewing service's edition, plan and device support. Finding a film on MovieFlixer does not guarantee that it has a 4K option in every region. Older restorations can look excellent, while a larger pixel count does not automatically mean a better master.
Persistent buffering
Try a lower available setting in the actual player and check whether other network activity is competing for the connection. MovieFlixer's catalog does not control that player's video quality. The playback help page separates a loading problem from a search problem.