when do brake pads need replacing? on 2002 Lexus LS430

front brake pads are at 4mm. When do they NEED to be replaced? Thanx.

Asked by for the 2002 Lexus LS430
Your better off changing them early because the inside pad could be worn out and just because the outer pad has 4mm doesn't mean the inside pad has the same measurement and if it destroys the rotors due to waiting too long this could be very expensive. The price for each rotor is $150 each for aftermarket and $222 each from the dealer