Hmm? I think I have found the problem as to why some Mp4 players are not recognized by your computer even though it is still able to charge. So after analyzing my usb cable that I got with my mp4, I notice that the end where you insert into your mp4 twists around very easily. I then look at the one I got with my camera and it doesn't twist at all. So I thought that a wire might have come loose. So I open the end where you insert into the mp4 with a x-acto knife and see that the blue wire has come loose . I re solder it back in place and use some super glue to put casing back together and now my computer recognizes my mp4 again.
But if your usb cable is defective i would suggest getting a new one instead of fixing it, risking your house or mp4 player getting blown up or something.