I have a video in MP4 format, taken on some type of mini video camera, possibly flip. Quicktime and VLC won't open it because they don't recognise it. Is this a codec issue or is the file corrupt? How can I tell, or how can I make it play?
MP4 is a container format and could contain many different codecs, DivX, Xvid, AAC, H.264 (or the "Mac" format that @Joe referred to).
You need to discover which codec is contained within in order to know how to play this format.
As others have mentioned, MP4 is a container format, and can contain any of several different video or audio codecs.
Both QuickTime and VLC should open MP4 easily. (Unless you have very old versions.) If you have recent versions and they aren't opening this file, the only possibilities are:
You could try checking what vid/aud codecs the file reports with Gspot. You may be able to identify the codec used and google for an installer. The newest Gspot version can tell you a
It's possible the camera uses a proprietary codec that is installed with a driver package that came with the camera, but I don't think this is common.