Absolutely legal. MCT varies by airline, but from memory it is either 75 or 90 minutes for CX/AA. In practice I've never had it take longer than 15 minutes to get from T4 through security at TBIT, including 5 minutes for the walk over and 5 minutes at the CX check-in counter ... but I take the late flight (so TBIT is quieter then, much longer lines earlier in the evening). LAX has interesting lines -- huge at 6 am and at midnight, empty at 3 pm -- due to the flight schedules and timezone differences. While T4 lines circa midnight are huge, you won't have a problem being EXP even if you need ticketing(presuming the EXP desk is still open at LAX). I'd guess 10 minutes to clear the F security line at T4, at the most -- just be sure to stay away from the Y side.