IPod Touch 4th gen Battery Problems

This is my 1st support discussion so bare with me. I have an iPod touch 4th generation for at least a year now & I'm having battery issues. I've factory restored it twice one from a backup & one from factory reset but still having the problem. My iPod says its 100% charge plugged in the dock or by the PC but once I take it out it turns off sometimes I can get it to turn back on but with at least 50% battery power or I can't turn it on & have to use it plugged in (while I'm writing this message).what should I do to fix this:

1. Upgrade to 5th gen

2. New battery (how much AUS dollar)

3. Stuff it & just leave it

4. Other ideas

It would be sooooo nice for a good rep once but one I can do cause I'm only 14.

Thanks