The file extension, .mp4, .wmv, .mov, etc. really has no bearing on whether it can play or not. It should, but that is not the way it works. The file type is merely a wrapper around the media content. That media content can be encoded with lots of different encoding algorithms. If it is encoded with a scheme that Quicktime doesn't understand, then it won't play in Quicktime Player.
You might try installing the Perian plugin for Quicktime. It might have the codecs necessary to decode the media content.
Another option is to use Video LAN's VLC. It plays a lot of things.