If you encounter this file on social media, via direct message, or on a suspicious website, do not click it .
Ensure the file is actually an .mp4 and not an executable like .mp4.exe , which is a common trick to hide malicious programs. How to Create and Open an MP4 File
If you have already interacted with the file, use a reputable antivirus or a website security checker to scan your device for hidden threats.
If you must open unknown media, use updated, sandboxed players like VLC which patch known vulnerabilities quickly.