COMMUTER ACFT MAKING DSCNT FOR APCH FLIES BELOW ASSIGNED ALT. CTLR GIVES VECTOR TO AVOID CONFLICT.

Narrative

WHILE CRUISING AT 5000 FT MSL; FO (ME) SWITCHED TO COMPANY FREQ WHEN 10 MINS OUT (COMPANY POLICY). NORMAL ALT FROM LAX TO OXR IS 4000 FT MSL. AFTER SWITCHING FROM LAX APCH TO BURBANK APCH; I OBSERVED THE CAPT REDUCE PWR (I WAS STILL ON COMPANY FREQ THEN ATIS). I FLASHED 4 FINGERS TO THE CAPT QUESTIONING IF WE HAD BEEN CLRED TO DSND TO 4000 FT. NO VERBAL COM TOOK PLACE. I THOUGHT I SAW THE CAPT NOD IN THE AFFIRMATIVE; THEN SWITCH THE ALT SELECTOR TO 4000 FT. WE BEGAN OUR DSCNT; THEN; AFTER COMPLETING THE COMPANY CALL AND GETTING ATIS; I ASKED THE CAPT IF WE WERE CLRED TO DSND. HE SAID WE WERE GIVEN A SPD RESTRICTION OF 160 KTS. WE CONTINUED DSCNT. AT 4500 FT BURBANK GAVE US AN IMMEDIATE CLB TO 5000 FT AND R TURN TO 360 DEG. A CONFLICTING SMA WAS AT 4000 FT. WE CLBED BACK TO 5000 FT AND REINTERCEPTED THE LOC FROM THE N. THE REST OF THE FLT WAS UNEVENTFUL. THIS INCIDENT RESULTED FROM A BREAKDOWN IN CREW COORD. THE FO (MYSELF) SHOULD HAVE BEEN CONCENTRATING ON HIS OWN DUTIES WHEN OFF THE ACTIVE ATC FREQ; COMBINED WITH A VERY HVY WORKLOAD (IMC WITH TURB AND LOTS OF TFC; AND AN AIRBORNE TIME OF 12 MINS FROM LAX TO OXR). A DESIRE TO HELP THE CAPT RESULTED IN A VERY EMBARRASSING AND POTENTIALLY VERY SERIOUS CONFLICT.