As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
PILOT REPORTED VIBRATION ON LEFT ENGINE. COMPRESSION PERFORMED AND FOUND CYLINDER NR 4 LOW. CYLINDER REMOVED AND FOUND EXHAUST VALVE SEAT LOSE AND PISTON DAMAGED BY THE VALVE. CYLINDER REPLACED. REPAIR STATION NOTIFIED ABOUT THE INCIDENT AND THEY ARE GOING TO MAKE AN INVESTIGATION.