Gorynel -
This is a touchy subject for any public forum. Here is what I can suggest, where you might find something useful.
In your favorite search engine, use these keywords: mp4 apple protected
You might want to carefully read the Wiki entry on this subject. Bottomline, Apple is very protective of their technology, whether it be software or hardware.
Regarding specific suggestions, I personally do not have experience since I rarely have songs in MP4 format, but from my result of my 'net search with the keywords above, it does appear there may be some solutions.
Good luck.
