As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CRUISE/CLIMB PERIOD OF FLIGHT, FLIGHT CREW FELT A LOSS OF RPM AND ENGINE ROUGHNESS/VIBRATION. FLIGHT CREW TERMINATED FLIGHT AND LANDED AIRCRAFT. MAINTENANCE PERSONEL TROUBLESHOT AND INSPECTED IGNITION/SPARK PLUGS, FUEL SYSTEM FOR DEFECTS. NO DEFECTS NOTED. PERFORMED DIFFERENTIAL PRESSURE CYLINDER LEAK CHECK AND DISCOVERED CYLINDER #4 WAS LEAKING BY THE INTAKE VALVE.