I/ For Window (Window XP, Vista, 7, 8…)
If you have yet to play mp4 file, gallwch install additional K-Lite Codec Pack
The K-Lite Codec Pack is a collection of DirectShow filters, VFW/ACM codecs, and tools. Codecs and DirectShow filters are needed for encoding and decoding audio and video formats. The K-Lite Codec Pack is designed as a user-friendly solution for playing all your audio and movie files. With the K-Lite Codec Pack you should be able to play all the popular audio and video formats and even several less common formats.
Lawrlwytho K-Lite Codec Pack
II/ For Mac
Using QuickTime Player