A poster is not a video sample
Artwork and the film stream are different files. A crisp image on HQFlix does not establish the resolution available through a viewing service. Read the offer details and inspect the player's supported formats for the particular edition.
Check the whole setup
A 4K screen is only one part of a 4K viewing setup. The app, selected plan, device, connection and any connected display equipment must also support the format. HDR is a separate capability. If the image looks unexpectedly dim, check the relevant display mode before increasing every brightness setting.
Stability can matter more than a number
When a player supports automatic bitrate adjustment, the image can change as the connection settles. A stable 1080p edition may be preferable to repeated interruptions at a higher setting. Black bars may preserve the film's composition, so inspect fit and zoom controls before treating them as a defect.