When in ProSelect on Windows, you are unable to see the .mp4 music files listed under the "Music Folder" option (Edit -->Preferences --> Slideshow & Music --> Music) , it's probably because you cannot play them outside the iTunes player.

In ProSelect,

1. If selecting an iTunes playlist as your music option, check you have the correct Playlist selected.
2. If selecting a Music Folder, then ProSelect uses Windows Media Player to play your music and will only be able to read .mp3 format music files.

You can use Media Player or iTunes to convert your .mp4 music files to .mp3.  For the iTunes conversion, refer to this article on Converting iTunes Tracks To MP3 Format.

If using Windows Media Player, under the Tools menu,

1. Choose Option --> Rip Music (tab),
   
2. Set the Rip music location (where your .mp4 music file are located)
   
3. Select .mp3 format in the drop down, then OK 
4. Make sure you have a bank CD loaded and choose Rip from the main menu.
   

You should be able to view the newly created .mp3 music files in the ProSelect music folder once you've copied the files to the folder where ProSelect is set to find them (ie. back in the original .mp4 files are). 

Remember to do this only if it is legal for you to do so.