I think your confused. MP4 is a container, not a codec. It usually contains an h.264 video stream as well as audio. Wikipedia for the long explanation if you like.
Originally Posted by John Private
As for playing back video of the type, I just install VLC, comes with everything needed. It's in the repos, if not, install all the restricted extras and you will get the codecs needed for totem to play. To use the following commands, please open a terminal (Applications > Accessories > Terminal) type em and push enter.
To install all the codecs and extras :
To install VLC
sudo aptitude install ubuntu-restricted-extras
You can of course do both, that works equally well, then you have multiple options. If you need more info to understand installing, see my sig, an old tut I wrote but still valid.
sudo aptitude install vlc vlc-plugins-pulse