The description below lists all compression and file formats that are on-topic here. Related categories would be MP3 (being a valid format in the MP4 container) or the Visual and Systems parts of the MPEG-4 standard (choose the general MPEG category for now), also DivX which is often used as a label for hardware MPEG-4 video players. So please submit your site in those categories if it better fits in there.
Since MPEG-4 is a comprehensive multimedia standard, this category lists sites about several audio formats, all being part of MPEG-4 Audio.
"Natural" coding of sound: Advanced Audio Coding (AAC, also its MPEG-2 version), TwinVQ (aka VQF), CELP, Parametric Codecs (HVXC, HILN).
"Synthetic" coding of sound: Structured Audio Orchestra Language (SAOL) and Structured Audio Score Language (SASL), Text-To-Speech (TTS).
The container format MP4 also belongs to this category, as long as it includes audio-only tracks with *.m4a, *.m4b or *.mp4 file extension and not video or other parts of the MPEG-4 standard like 2D/3D graphics.
Due to the new developments in mobile communication the 3GPP/3GP2 standard and its audio codecs (several AMR and AAC variants) should be included here, also because their container formats *.3gp and *.3g2 are very similar to MP4, although not the same.