There exists many MP4 demuxers but only a few MP4 muxers. And most of them are not interoperable.
The MP4 standard is architectured around a core specification called Iso Base Media File Format (ISOBMF, MPEG4 part 12) and several interoperability standards (such as MPEG4 part 15 for H264/AVC integration) and then recommandations. It is a full-time job to keep updated.
GPAC goes even further as it serves as a reference implementation and experimentation platform in the standardization field. We feature MP4Box, our packaging tool, and a full playback infrastructure.