As reported to the FAA. Reports are unverified, and codes are left as the FAA publishes them.
What was found
DURING CLIMBOUT FROM ONT A/C HAD FIRE WARNING WITH CORRESPONDING NR 4 MAIN CARGO LIGHT ILLUMINATED. PERFORMED QRH & RETURNED TO ONT. SMOKE LIGHT REMAINED ON UNTIL PULLING INTO THE ONT RAMP. VISUAL INSPECTION AFTER LANDING REVEALED NO INDICATION OF FIRE OR SMOKE.