What was found
AIRCRAFT WAS GROUNDED: WHILE IN CRUISE AT FL 360 FOR APPROXIMATELY 1 HOUR THE #4 MAIN BRAKE TEMPERATURE INDICATION GRADUALLY INCREASED TO OVER 400'C AND THEN FAILED WITH "XX". COMPLIED WITH ECAM, BUT SHORTLY AFTER #3 MAIN PRESENTED SIMILAR INDICATIONS INCREASING OVER 400*C AND THEN FAILING. AN EMERGENCY WAS DECLARED AND THE AIRCRAFT DIVERTED TO LAS WHERE IT LANDED SAFELY WITH OUT FURTHER INCIDENT. MAINTENANCE REPLACED THE RIGHT MAIN LANDING GEAR BRAKE TEMPERATURE MONITORING UNIT AND ACCOMPLISHED A SATISFACTORY OPERATIONAL CHECK PER AMM 32-47-18.