What program are you using?
I'm pretty sure if you google youtube downloader, you have the option of downloading the MP4 file (Video & Music)
Basically go to the video you want, copy the link into the download bar and click download and it'll do everything for you.
^ Thats for the pro version.
I THINK you can download the mp3 file, then convert it to mp4
Edited once, 2/2/12 - 12:23pm.
Posted on Thursday, 2nd February 2012