THE NO 4 ENGINE CAUGHT FIRE DURING CLIMB AFTER TAKEOFF AND THE CREW ACTIVATED THE ENGINE FIRE EXTINGUISHER SYSTEM, DUMPED FUEL, AND RETURNED TO THE ARPT FOR AN EMERGENCY LANDING. THE NO 4 ENGINE FIRE AND A GRASS FIRE AT THE RWY T/D ZONE WERE EXTINGUISHED BY ARPT CFR CREW. AN EMERGENCY EVAC WAS CONDUCTED FROM THE LEFT SIDE AFTER THE ACFT STOPPED ON THE RWY. THE L5 EMERGENCY DOOR AND SLIDE FAILED TO DEPLOY FOR THE EVAC. EXAM OF THE ENGINE REVEALED EVIDENCE OF FUEL LEAK FROM THE MAIN FUEL CONTROL UNIT.