MD88 CREW ACCEPTS CLRNC TO DESCEND TO 4000 FT. AT 5200 FT; PHL TRACON ADVISES THEY WERE CLRED TO 6000 FT NOT 4000 FT.

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WHILE ENROUTE TO PHL WE RECEIVED THE LATEST ATIS INFO ADVISING THAT THE WINDS WERE 04013G18KT AND TO EXPECT VISUAL APCH TO RWY 9R. WE SET UP THE FMS FOR RWY 9R AND I BRIEFED THE CAPT FOR A VISUAL APCH. IN THE ARR WERE ASSIGNED A DESCENT FROM 10000 FT TO 4000 FT AND ASSIGNED A HEADING OF 040 DEGS. THE CAPT READ BACK THE CLRNC AND THE TWO OF US CONFIRMED THE ALT WAS SET ACCORDINGLY IN THE AIRPLANE. WHILE DESCENDING THE APCH CTLR ADVISED US TO EXPECT A VISUAL APCH TO RWY 27R AT PHL. ACCORDING TO THE ATIS THE WINDS WERE REPORTED AT 04013G18KT. I USED THE ACARS TO SEND FOR THE LATEST ATIS; WHICH NOW ADVISED THAT RWY 27R WAS IN USE AT PHL. THE CAPT AND I DID NOT THINK THAT A VISUAL APCH TO RWY 27R WOULD ALLOW US TO LAND AND THE CAPT STARTED WORKING OUT THE TAILWIND COMPONENT FOR THE LNDG RWY BASED ON THE RPTED WINDS. I QUERIED ATC ABOUT THE WIND AND THE CTLR STATED THAT THE WINDS WERE NOW 03012KT. THE CAPT CONTINUED TO COMPUTE THE TAILWIND COMPONENT TO VERIFY THAT WE WOULD NOT EXCEED THE LIMITATIONS OF THE ACFT FOR LNDG. I RESET THE FMS FOR AN APCH TO RWY 27R TO SET THE NAV RADIOS TO APPROPRIATE FREQUENCY. THE CAPT DETERMINED THAT THE TAILWIND COMPONENT WOULD NOT EXCEED THE LIMITATIONS OF THE ACFT AND WE RE-BRIEFED THE APCH. AS WE WERE DESCENDING THROUGH 5200 FT; THE NY CENTER CTLR QUERIED US ABOUT OUR CURRENT ALT. THE CAPT REPLIED THAT WE WERE DESCENDING THROUGH 5200 FT FOR 4000 FT. THE CTLR IMMEDIATELY RESPONDED BY ADVISING US THAT OUR ASSIGNED ALT WAS 6000 FT AND TO STOP OUR DESCENT. WE LEVELED OFF AT 5200 FT AND CONTINUED ON THE 040 DEG HEADING. THERE WERE NO FURTHER COMMENTS ABOUT THE DIFFERENCE IN ALTITUDES FROM ATC AND HE CONTINUED TO VECTOR US FOR THE VISUAL APCH TO RWY 27R. THE APCH AND LNDG WAS COMPLETED WITHOUT ANY OTHER DISCREPANCIES. NONE OF THE ATC CTLRS ADVISED US TO CONTACT ANYONE AND NOTHING ELSE WAS SAID ABOUT THE ALT DEV. CONTRIBUTING FACTORS TO THIS ALT DEV WERE AS FOLLOWS. WE WERE FINISHING THE LAST DAY OF A 4 DAY TRIP. WHICH IN THE LAST TWO DAYS CONSISTED OF TWO REDUCED REST OVERNIGHTS WITH A 14.5 HOUR DUTY DAY IN BETWEEN THE REDUCED REST PERIODS. THE ORIGINAL ATIS AT PHL REPORTED THAT WE SHOULD EXPECT A VISUAL APCH TO RWY 9R BASED ON THE WINDS AND THEN WE WERE ASSIGNED RWY 27R; ACCORDING TO AN UPDATED ATIS REPORT AND ATC THE CHANGING OF THE EXPECTED RWY AT PHL CREATED CONFUSION BASED ON THE REPORTED WINDS AND CAUSED US TO BE DISTRACTED BELOW 10000 FT. ATC DID NOT CORRECT US WHEN WE READ BACK THE CLRNC OR WE MISUNDERSTOOD THE CLRNC AND SET THE ALT IN THE ACFT TO 4000 FT BY MISTAKE; WE ARE NOT SURE OF WHICH ONE OF THESE THINGS HAPPENED. IN THE FUTURE; THIS TYPE OF OCCURRENCE COULD BE PREVENTED BY HAVING CONSISTENCY IN WHICH ARPT OPS ARE BEING CONDUCTED. ACFT SHOULD NOT BE MADE TO LAND WITH A TAILWIND COMPONENT OF THE STRENGTH THAT EXISTED AT PHL ON THIS DAY; SINCE IT CAUSED THE CAPT AND I TO BE CONCERNED ABOUT EXCEEDING THE LIMITATIONS OF THE AIRPLANE. ALLOWING ADEQUATE TIME TO REST WITHOUT SUCH LONG DUTY PERIODS IN BETWEEN REDUCED RESTS; MAY ALSO HELP PREVENT SUCH THINGS FROM OCCURRING. PROPER 'CHECKS' IN COMS BETWEEN PLTS AND ATC WOULD HAVE ALSO PREVENTED THIS OCCURRENCE FROM HAPPENING.