LOST R GEAR #4 TIRE AND WHEEL ASSEMBLY ON LNDG.

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LANDED RWY 07 SBA. SMOOTH TOUCHDOWN AND ROLLOUT. AT 90 KTS CAPT APPLIED BRAKES; A THUMP WAS HEARD IN THE COCKPIT AND THE OUTBOARD ANTISKID INOP LIGHT ILLUMINATED. WE TAXIED TO THE GATE AND SHUT DOWN. WE THEN LEARNED THAT THE #4 TIRE (R GEAR OUTBOARD) AND THE WHEEL ASSEMBLY WERE MISSING FROM THE ACFT. I CALLED GND SO THEY COULD CLR THE DEBRIS FROM THE RWY. THEY FOUND THE BRAKE ASSEMBLY THAT NIGHT BUT THE TIRE WAS NOT RECOVERED UNTIL MORNING -- ON THE SIDE OF THE RWY. THE MAIN AXLE BTWN THE 2 MAIN GEAR TIRES BROKE IN 1/2 DURING NORMAL BRAKE APPLICATION. THIS WAS OUT SIXTH LEG OF THE DAY; OUR FIFTH IN THIS ACFT. WE THOUGHT AS WE TAXIED IN THAT MAYBE WE HAD BLOWN A TIRE. MAINT SAID THIS AIRPLANE HAD AN OVERWT LNDG 3 DAYS BEFORE; BUT ONLY 1000 POUNDS OVER MAX LNDG WT. THERE WAS NO EVIDENCE OF ANY HIDDEN STRESS CRACKS AROUND THE AXLE PIECES THAT WERE RECOVERED. WE BELIEVE ALL PARTS WERE FOUND. AT TIME OF LNDG WE WERE 4600 POUNDS PLUS UNDER MAX LNDG WT FLAPS 25 DEG CALM WINDS AND VFR. NO OTHER DAMAGE WAS DONE TO THE ACFT; PAX OR CREW.