You can burn an MP4 music file to CD by inserting your blank disk into your computer's disc tray and opening the drive in the "Devices with removable storage" section under the "Computer" option from the Start menu. You'll then need to open the folder containing the file you want to burn and drag and drop it into your disk drive's window. Files will burn automatically if you're using the "Live file system format" once you've formatted and named your disc.
If the MP4 file you want to burn is in your iTunes library, insert your disc and click on "New playlist" from the "File" menu in iTunes. Drag and drop the file you want to burn from the main iTunes library to the new playlist window and click on burn at the bottom of the screen.
If the file you are attempting to copy is DRM protected, you might only be able to burn a limited number of discs or not be allowed to transfer to CD at all.