I have too many and they are too large to email.
There is already a lot of good firmware's on this site but you need to open them up with the resource editor to at least check that they are for the same screen size and have a similar menu layout.
Have you tried
http://www.uxcell.com/mp3mp4-players-c-981.html
Check their products first for model codes and then go to their download pages and select the firmware.
Do you have some details of your MP4 Player, photo's or at least a link to eBay with a similar player?
Cheers, Douglas