CTLR INADVERTENTLY ASSIGNED WRONG ALT. ON HDOF NEW CTLR CORRECTED ALT PROB.

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I WAS THE CAPT AND PF ON APCH TO LGA. THE COPLT (PNF) WAS OPERATING THE RADIOS AND TALKING TO ATC. WE WERE BEING VECTORED FOR THE ILS RWY 4 APCH TO LGA. WE WERE TOLD TO DSND TO 4000 FT AND PROCEED DIRECT TO GRENE INTXN. AS WE WERE DSNDING WE WERE THEN TOLD TO CONTACT LGA APCH ON ANOTHER FREQ. AS WE WERE PROCEEDING DIRECT TO GRENE INTXN WE WERE DSNDING THROUGH 6500 FT FOR 4000 FT WHEN THE CTLR TOLD US WE WERE SUPPOSED TO MAINTAIN 7000 FT. THE COPLT THEN TOLD THE CTLR WE WERE CLRED TO 4000 FT BY THE PREVIOUS CTLR. WE WERE THEN TOLD TO MAINTAIN OUR PRESENT ALT OF 6500 FT. WE LEVELED OFF FOR APPROX 10-15 SECONDS; THEN EWR WERE TOLD TO DSND TO 4000 FT AND TURN TO HDG OF 060 DEGS TO INTERCEPT THE LOC FOR RWY 4. THERE WAS NO TFC CONFLICT AND THE APCH WAS CONTINUED AND THE LNDG WAS MADE WITHOUT INCIDENT. ASAP I CONTACTED NEW YORK TRACON ON THE TELEPHONE AND SPOKE TO THE CTLING SUPVR. THE SUPVR PUT ME ON HOLD FOR A FEW MINS; CAME BACK; AND TOLD ME THERE WAS NO PROB; AND AS FAR AS THEY WERE CONCERNED EVERYTHING WAS FINE. I FEEL THE CTLR POSSIBLY GAVE US A LOWER ALT THAN THE NEXT SECTOR WAS ANTICIPATING. I DO NOT FEEL THAT MY CREW WAS IN ERROR.