FLC OVERSHOOTS ASSIGNED ALT.

Narrative

AFTER DEPARTING PIE; WE WERE GIVEN ALT INITIALLY OF 1600 FT. WE LEVELED OFF UNTIL REACHING DEP CTL WHO THEN CLRED US HIGHER. ALTHOUGH I HEARD THE CLRNC FROM DEP CTLR; I DID NOT HEAR THE READBACK DUE TO THE RADIOS ON THIS ACFT. (I COULD NOT HEAR MY COPLT'S XMISSIONS.) FOR SOME REASON I FOCUSED ON THE 1600 FT AND KEPT 6000 FT IN MY MIND FOR NEXT ALT. AS I'M RAPIDLY CLBING THROUGH 5000 FT; I SORT OF HAD A FLASHBACK THAT MADE ME QUESTION MY COPLT AS TO OUR ASSIGNED ALT AND HE THEN SAID 4000 FT WAS THE ASSIGNED. I IMMEDIATELY BEGAN A DSCNT BACK TO 4000 FT AND AT THIS POINT ATC ASKED US TO SAY ASSIGNED ALT. WE CONFIRMED TO HIM WE WERE 4000 FT AND CORRECTING. HE SAID 'ROGER' AND THAT WAS THAT. SHORTLY THEREAFTER WE WERE CLRED HIGHER AND HANDED OFF. ONCE AGAIN; ALT ALERT SHOULD BE A MANDATORY PIECE OF EQUIP; ESPECIALLY IN COMMUTERS WHERE WE OFTEN BY 8-10 LEGS A DAY AND FATIGUE CAN CAUSE ONE TO PERHAPS NOT BE AS CONCENTRATED AFTER 12-13 LEGS IN 2 DAYS. USUALLY; AFTER GOING TO DEP ON THIS RTE; WE'RE CLRED FROM 1600 FT AND THEN UP TO 10000 FT OR HIGHER. A CREATURE OF HABIT -- I FOCUSED ON THE HIGHER ALT. BETTER COM BTWN CREW AND ALT ALERT COULD HAVE PREVENTED THIS. NO OTHER ACFT CONFLICTS WERE INVOLVED.