Codecs for os x Mountain Lion

Exactly what codec are you asking about? They are built in.

Is there a particular video you are not able to watch? You can download VLC which will play most anything.

Well, I don't have an answer for you - I just tried one and it worked without a problem.

MPEG-4 is a container; so a couple of things come to mind: what format is the actual video? And, could there be any copyright or DRM issues involved? Is this a homemade video or commercial movie?