So, it is not M4 as in the title.
Still, read the manual.
J-12 might benefit from wide hood sold on eBay.
Add cable tie as focus tab, it will make huge difference.
Nothing wrong with this lens on film. Add filter and hood for easier aperture adjustment.
Serial number story is old myth, all of the problems were sorted out already, those cameras are not shelf queens.
It camera works no need to send it for service.