Depends what you mean by mpeg4. Osmo4 is a free player that supports mp4 playback including ttext subs and advanced interactive stuff like menu's. There is a also a player that comes with mpeg4ip as well as mplayer and VLC. Plus any directshow player if you have Nero's filters installed or a suitable splitter and decoders. 3ivX have a full set of filters for ASP MPEG4 mp4's. Moonlight also have a splitter which can handle both MPEG4 and AVC. Then there is haali's matroska splitter which can also handle mp4's, including ttext subs and I think Nero's vobsubs. For decoding ffdshow supports mp3 and aac audio as well as SP/ASP MPEG4 and AVC video. So far from not existing. There are many alternatives, I guess you didn't search too good.
