The aircraft was on climb passing through 1200 ft, when the crew observed a fire warning on the right outboard (No. 4) engine. Following completion of the checklist items, the engine was shut down. The aircraft returned to Adelaide and conducted an overweight landing. Engineering inspection of the No 4 engine, found that an "O" ring in the combustion chamber fuel manifold at the No 7 fuel nozzle, had split internally. The resultant small fuel leak and fire had triggered the fire warning. The detector wire is approximately 3 inches downstream of the failed ring. As the engine was approaching a 5,000 cycle heavy inspection, an engine change was carried out and the aircraft returned to service. The fuel nozzle "O" ring has a finite life of 5,000 cycles. The failed ring had completed 4673 cycles and is the subject of a Defect Report submission to CASA.