An MP4 is just a container. Inside the container is an audio file, a video file and maybe a subtitle file. What you want is to extract the audio portion from the container. Once you've extracted it, you don't need to convert it to edit it. It will likely be an MP3 file or an AAC file which can be edited in Audacity without converting it to wave.
MP4 is usually the preferred format for uploading videos to YouTube, Vimeo or other websites. MP4 videos are made up of video and audio elements that are compressed separately. The video part is usually compressed with MPEG-4, and the audio is usually compressed using AAC, which is the audio format associated with iTunes. The imported file always appears in a new track in that project. 1. File > Import > Audio. If you select the File > Import > Audio command, then choose one or more audio files, Audacity will import the selected file (s) into the existing project. This is useful to bring the content of one or more audio files into a project that already contains This tutorial video will show you step by step how to burn MP3 files to an Audio CD for any CD player and car stereo using Windows Media Player in Windows 7,