As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE, THE PILOTS NOTICED THE NR 4 ENGINE STARTING TO BACKFIRE, THE BACK FIRING WAS GETTING OUT OF CONTROL SO THE ENGINE WAS SHUT DOWN AND THE PROPELLER FEATHERED. DURING THE INSPECTION OF THE NR 4 ENGINE IT WAS FOUND THAT THE FRONT AND REAR SPARKS FOR THE NR 2 CYLINDER WERE FOULED. THE PLUGS WERE REPLACED AND ENGINE RUN-UP WAS FINE.