Starter clicks, dsn’t turn engine. Tried rotating engine, engine will not turn past a certain angle. Removed spark plugs for cylinder inspection. 1,2, and 3 ok. Cylinder 4 spark plug extremely difficult to remove. Ground strap has been impacted by debris. Borescope inspection shows two broken off exhaust valves laying on the piston. Valve cover removed to inspect timing chain and sprokets. Chain and sprokets are intact. Engine reassembled besides cylinder 4 spark plug and coil pack.