I/ For Window (Window XP, Vista, 7, 8…)
Download GomPlayer: http://player.gomlab.com/eng/download/
If you have yet to play mp4 file, you can 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.
Download K-Lite Codec Pack: http://www.codecguide.com/download_k-lite_codec_pack_basic.htm
II/ For Mac
Using QuickTime Player