BE1900 ASSIGNED 4000 FT TOLD TO EXPECT A VISUAL. CAPT THOUGHT HE WAS CLRED FOR A VISUAL AND GOT TO 3800 FT WHEN ATC CORRECTED HIM.

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ON APCH TO PDX; WE WERE ON A VECTOR OF ABOUT 350 DEGS AND AN ALT OF 4000 FT. WE WERE THEN CLRED FOR THE VISUAL APCH TO RWY 10L. AS I BEGAN MY DSCNT; APCH CALLED AND ASKED WHAT OUR ALT WAS. THE FO RESPONDED WITH 3800 FT AND APCH THEN SAID WE WERE SUPPOSED TO BE AT 4000 FT AND HAD NOT BEEN CLRED FOR THE VISUAL. I RESPONDED THAT I THOUGHT WE HAD BEEN (WHICH I BELIEVE TO BE TRUE) AND APCH THEN CLRED US FOR THE VISUAL. NO FURTHER ITEMS OCCURRED RELATING TO THIS FLT. THERE WRE NO OTHER CONFLICTS. SUPPLEMENTAL INFO FROM ACN 368337: PDX APCH HAD CLRED US FROM 5000 FT MSL TO 4000 FT MSL AND TOLD US TO EXPECT THE 'COMMUTER VISUAL' (LCL PROC FOR VISUAL APCH). APCH ASKED US TO KEEP OUR BEST POSSIBLE SPD UP FOR INCOMING TFC FROM THE N AND TO CONTINUE TOWARD THE PWRLINES. WE THEN RPTED THE FIELD IN SIGHT AND CONTINUED TOWARD THE PWRLINES WHICH PUT US ON A R BASE TO FINAL TO LAND ON RWY 10L. THE CAPT BEGAN TO DSND TOWARD THE PWRLINES; AT 3800 FT MSL APCH ASKED US OUR ALT AND WE SAID '3800 FT.' HE THEN REPLIED; 'I THOUGHT YOU WERE ASSIGNED 4000 FT' AND WE RESPONDED 'WE THOUGHT WE WERE CLR FOR THE COMMUTER VISUAL.' WE THOUGHT WE HAD BEEN CLRED FOR THE COMMUTER VISUAL WHICH MEANT WE COULD DSND TO 2000 FT MSL UNTIL XING THE PWRLINES.