: Occasionally, a file labeled .mp4 is actually an .mkv or .ts . Try changing the extension to see if it triggers the correct playback engine. To give you the best advice, could you tell me: What error message do you see when you try to open it?

A highly-rated tool on the Microsoft Store that can fix sync issues, header corruption, and frame damage for 4K MP4 files.

FFmpeg is the Swiss Army knife of video repair. This method rewraps the file without re-encoding (lossless).