Chris Graham

It's very very complicated.

MPEG is like a commitee, and they have various standards. The problem is, with MPEG 4 everything went to hell and all different vendors created their own variations (codecs), used different file type extensions, and different 'container formats'. Forget even trying to think of something as being 'MPEG' - it's the combination of the container format, the video codec, and the audio codec, that matter.

In this case .mp4 files seem to be predominantly an Apple convention, meaning they'll be using Apple's (latest) favoured codec. Renaming to/from .mpeg or .mpg should be ok, but it is likely also a fairly meaningless operation.