If you think the little USB charger that came w/ your MP4 is not working, here's a great replacement:
Zip-Linq - ZIP-PWR-X2 - $6.99 at Buy.com, eligible for free ship. This is the best price I've seen on these, especially if you can meet the $25 minimum & get free shipping.
This gizmo lets you charge any USB-chargeable device from either a 120v household outlet, OR from a 12v car lighter-type socket.
So it's like getting a bonus car charger for your MP4 player for free.
If, on the other hand, your little USB/charger
cable is bad, you need to get a "USB Type A to
5-pin USB MINI Type B". These are fairly easy to find on eBay for just a couple dollars.
Make sure it's a 5-pin (or whatever # of pins your player's connector may have). Most MP4 players out there use a 5-pin, but there are a few that use 4-pin.
I hope this all helps.
JohnB