What was found
(CAN) DURING TAXI OUT, THE ACFT FOLLOWING NOTED THAT THIS ACFT APPEARED TO HAVE A FLAT TIRE ON THE RT SIDE. THE ACFT RETURNED TO THE GATE BUT STOPPED SHORT OF THE GATE AS THEY COULD MOVE NO FURTHER DUE TO THE NR 4 BRAKE BEING SEIZED. INVESTIGATION FOUND THE NR 4 BRAKE UNIT WARPED AND SEIZED FROM OVERHEATING. THE NR 4 MAIN WHEEL FUSIBLE PLUGS HAD RELEASED ALLOWING THE TIRE TO DEFLATE. THE NR 3 MAIN WHEEL TIRE FAILED DURING THE RETURN TAXI. THE NR 4 BRAKE UNIT AND BOTH WHEELS WERE REPLACED. THE ANTI-SKID SYS HAS BEEN TESTED WITH NFF.