What is this "mp4" player you are getting? Anyhow, it might be able to play some of your files from iTunes. However, most tracks you can buy on iTunes will not play on anything but an iPod, the only way to work around this is in new versions of iTunes to burn them to a CD then rip them again.
If you have not bought any music on iTunes, or have only bought iTunes Plus tracks, this will not be a problem.