A sharp poster is not a resolution label
The image on a title page helps you recognize a film. It does not certify the resolution of a viewing offer. Open the offer and check whether it lists HD, Full HD, UHD or a specific resolution. A service can carry the same film in different formats depending on your plan, device or country.
1080p describes a picture size, while 4K usually refers to a larger UHD picture. Compression, source quality, contrast and display settings also shape the result. A well-presented older film can look more natural than an aggressively processed image with a bigger number attached.
Start with the screen you will use
A phone on a limited connection may suit a lower resolution. A large television makes detail more visible but also exposes compression. HDR and surround sound have their own support requirements; neither is guaranteed by the word 4K. Read the edition information before choosing a plan for one film.
When quality changes during playback
Some destination players adjust bitrate automatically as connection conditions change. If the picture stays soft, check the quality setting and give playback time to stabilize. Test another supported device before assuming the title itself is defective. MyFlixtor has no on-page resolution switch because playback controls belong to the viewing service you open.