MP4 is not a codec, it is a container. Different video codecs can be used in MP4 files including MPEG-4 (DivX, XVid,...), MPEG-4 AVC (x264),...

To watch MP4 as well as other most popular videos on your PC, I suggest you to install VLC player. It doesn't need any external codecs.

Also I used to install K-Lite Mega Codec pack. It allows you to watch MP4 and other the most popular video in Windows Media Player, Media Player Classic as well as many other players. The problem is that codec packs can cause problems to video editing software, video players,...