What was found
NR 4 ENGINE FIRE INDICATION. CREW REPORTED NR 4 ENGINE FIRE INDICATION AND BOTH FIRE BOTTLES DISCHARGED. PASSING THRU FL310 HEAD MUFFLED THUD FROM RIGHT SIDE, NO OTHER INDICATIONS. SHORTLY AFTER NR 4 ENGINE FIRE SWITCH ILLUMINATED 30 SECONDS LATER FIRE BELL RANG. ACCOMPLISHED ALL EMERGENCY AND APPLICABLE CHECKLIST'S. REMOVED AND REPLACED LT AND RT SIDE COWLS IAW MM 71-11-00 OP'S NORMAL. REMOVED AND REPLACED NR 4 ENGINE TURBINE EXHAUST SLEEVE (NOZZLE) IAW MM 78-11-02 OP'S NORMAL. REMOVED AND REPLACED NR 4 ENGINE FIRE BOTTLES PER 747 MM 26-21-03. REMOVED AND REPLACED NR 4 ENGINE PER EIA WORKCARDS F7103, F7201-JT9, F7204, F7205, F7207, F7209.