What was found
PILOT REPORTED, SHORTLY AFTER T/O FIRE WARNING BELL AND LIGHT ACTIVATED FOR APPROX .5 SECOND. DUE TO THE SHORT DURATION OF WARNING, COULD NOT PINPOINT WHICH ENGINE BUT JUMPSEAT MECHANIC SAID DEFINITELY NR 3 OR NR 4 FIRE WARNINGS TESTED AND ALL CK OK. NR 4 ENG COMPARISON RUNS PERFORMED UP TO TAKEOFF POWER, COULD NOT DUPLICATE FIRE WARNING DISCREPANCIES. REPLACED A7 AND A11 FIRE WARNING CARDS, SYSTEMS OPS CHECKED GOOD IAW MM 26-11-00.