In most scenarios mp3 to mp4 conversion is related to a simple encoding of MP3 audio to MP4 format, something you can nowadays easily achieve using plenty of available multimedia converters, such as the ones listed below. Considering it is one of the most common audio conversion, it should be pretty easy to convert mp3 to mp4.
Open MP3 file Open MP4 fileA multimedia converter
A free audio converter
A set of audio and ripping utilities for Windows
A multifunctional media converter for Windows
A music library management program - renamed to Helium (version 12) and replaced in 2016
A free multimedia converter in version for Mac
Universal video converting software for Mac
A multimedia player with conversion capabilities
Universal All Purpose Free Media, Audio & Video Converter Player Recorder
One of the many video conversion programs
Video converting software