this is a common problem in mp4 players with nokia battery as far as i have noticed.
i have a mp4 player which uses the bl-4c battery too and it gave me the same problem.
the solution is to replace the battery provided with the mp4 player with a genuine nokia bl-4c like i did and it worked for me. not only do they solve the restarting problem but since they are 600mAh or something like that, they give you much more battery life than the ones which come with the mp4 player and have 320mAh
when you are buying the battery, please put it in and try it in the shop itself to see if it works.
if it still doesnt, then its a problem in the player and has to be replaced in my opinion.
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