Use Handbrake (free) or Adobe Media Encoder (paid) to transcode the video.

Double-click to open in Windows Media Player or use the Microsoft Support guide to select a preferred player.

Use QuickTime Player or download VLC Media Player from the official site for broader codec support.

If you are trying to "put together" a guide for an event that isn't playing:

/var/cache/zoneminder/events/[Monitor_ID]/[Date]/62727/ Filename: 62727-video.mp4 2. Opening & Playing the MP4