This plugin decodes .MP4 and .M4A files as well as AAC, PS AAC and the newer HE ACC profile included in Nero 6.0 and can be considered a correctly working replacement for in_mp4 included with Winamp 5. It includes MP4 tagging.

Download MP4 Input Plugin

How to install :

- Uncompress in_mp4.dll from archive and copy it in Winamp \ Plugins directory.
- Always backup the existing file!

Important!

As from Winamp 5.12 and later, the default Nullsoft MPEG-4 Decoder (in_mp4) supports he-aac/nero digital mp4/m4a audio files, so there's no longer any need to use this plugin.
Note, the new in_mp4 still doesn't support mpeg4 video or proprietary Apple lossless audio (alac), though neither does the 3rd-party plugin. You'll still need to use the DirectShow Decoder for .mp4 video files; that's why you have to add ;MP4 to the Extension List in the DirectShow Decoder config (see the 2nd screenshot) by going to Preferences > Plug-ins > Input > double click "Nullsoft DirectShow Decoder...".
FFDShow may need to be installed to decode MPEG4 audio and video formats. 3ivx might be required in some cases.
WARNING: This will use the DirectShow filter for all MP4 files, therefore tags will not be read when adding MP4 audio files to the library and some audio formats in MP4 files might not function properly. You can circumvent this by renaming any .mp4 audio files with the .m4a extension, so that the Nullsoft in_mp4 decoder will be used for audio-only files. More info here.