As far as I know Windows Media Player dont play mp4 files...
You could use real player to play the mp4 files...
If you want to convert them to mpeg files or something to be able to play on media player try and download a program called super. Its free and converts to alot of different files and accepts tons of input files...
Here.
http://www.erightsoft.com/SUPER.html
On a side note Im from Rhode Island also..