What was found
CYLINDER NR 4 EXHAUST VALVE STUCK OPEN ON TAKEOFF. AIRCRAFT RETURNED TO AIRPORT AND MADE A SAFE LANDING. UPON INSPECTION, THE CYLINDER'S NR 4 EXHAUST ROTATER CAP WAS FOUND IN THE BOTTOM OF THE ROCKER BOX. INSPECTED VALVE PER LYC SB 388B. CLEARANCE FOUND TO BE SATISFACTORY. REMOVED CYLINDER FOR INSPECTION AT OVERHAUL FACILITY. GUIDE WAS REAMED, SEAT WAS GROUND. RE-INSTALLED CYLINDER ASSY. SUBMITTER SUGGESTED CAUSE AS CARBON 1 LEAD DEPOSITS STUCK VALVE OPEN.