This is usually a sign of CPU bottlenecking. Enable "Hardware Acceleration" in your media player settings to shift the load to your GPU.

Crucial Step: Avoid "Transcoding". If your server attempts to convert the file on the fly, it will degrade the 4K resolution down to 1080p and max out your CPU. Audio and Video Calibration Matrices

: Misconfigured operating system scale-and-layout percentages.

: To view the content in true 4K, you must have a 4K-capable monitor and a GPU that supports 4K output. 2. Digital Platforms