ACR ON DSCNT CLRED TO 8000 FT. ON READBACK TOLD 4000 FT. PASSING 6000 FT TOLD CLRNC WAS TO 8000 FT.

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WE WERE CLRED TO 10000 CONTACT ORD APCH INBOUND FOR VECTORS LNDG RWY 4R; I WAS FLYING AND MY FO TALKING. APCH CLRED US TO 8000 FT; MY FO READ BACK AND APCH EMPHATICALLY REPLIED 4000 FT! WHICH MY FO DUTIFULLY READ BACK AND SET IN THE ALT SELECT WINDOW. HE THEN ASKED FOR A REPEAT AND WAS TOLD 4000 FT AGAIN. I GUESS BTWN 6500 AND 6000 APCH ASKED WHAT ALT WE WERE PASSING AND 'WHAT ARE YOU DOING THERE; YOU'RE SUPPOSED TO BE 8000 FT?' BY THE TIME ALL WAS CONFIRMED WE BOTTOMED OUT AT 5500 BEFORE CLBING BACK TO 8000 FT. IT WAS A LITTLE UNNERVING -- SINCE THERE WAS A HVY LOAD OF INBOUND TFC AND A LOT OF CHATTER. WE ELECTED NOT TO ARGUE THE POINT -- A FOLLOWUP CALL TO THE TRACON SUPVR WAS MADE. HE SAID THERE WAS 'NO PROBLEM' (WITH AN ALT VIOLATION) AND THE CTLR GOT A LITTLE 'MIXED UP' AND THERE HAD BEEN 'NO CONFLICTING TFC.' THANK GOD! THE THOUGHT TURNS MY STOMACH! WE WERE ALL LUCKY THIS TIME. TRACON AT ORD REGULARLY ISSUES 5-6 INSTRUCTIONS AT ONCE IN A SINGLE BURST -- WITH NO TIME FOR ANYONE TO BREAK IN BTWN. I DO NOT ENVY THOSE CTLRS THEIR JOBS. WOULDN'T YOU SAY THAT AT TIMES THE VOLUME AT ORD EXCEEDS ATC CAPABILITY FOR SAFE OPS? I OFTEN WONDER ABOUT IT.