MPEG-4 files that only contain audio track often have M4A extension, so if you MP4s are only songs you can simply rename the file extension from MP4 to M4A and no mp4 to m4a conversion is actually required. If you MP4s contain movie and you just want the audio, you will have to use a proper multimedia conversion utility.
Open MP4 file Open M4A fileA DRM remover and media converter
A multimedia converter
A set of audio and ripping utilities for Windows
A music library management program - renamed to Helium (version 12) and replaced in 2016
A multimedia converter for formats suitable for iPad tablets
Universal video converting software for Mac
A multimedia player with conversion capabilities
Universal All Purpose Free Media, Audio & Video Converter Player Recorder