A B747 CREW SET THE ALTIMETERS TO 29.96 HG INSTEAD OF 996 HP AND WERE 400 FT LOW ON APCH. THE CREW WAS EXTREMELY FATIGUED.

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WHILE BEING VECTORED FOR THE APCH AND LNDG IN ZZZZ WE WERE INFORMED BY THE ZZZZ APCH CTLR THAT WE WERE BELOW THE CLRED ALT. THE CTLR RESTATED THE ALTIMETER SETTING AND DIRECTED OUR FLT TO CLB TO THE ASSIGNED ALT. AFTER SOME CONFUSION WE REALIZED THAT BOTH THE CAPT'S AND THE FO'S ALTIMETERS WERE STILL SET TO INCHES; WHICH RESULTED IN A DSCNT BELOW THE CLRED ALT. THE FLT DSNDED 400 FT BELOW THE CLRED ALT BEFORE THE ALTIMETERS WERE RESET TO HECTOPASCALS AND THE CLB WAS INITIATED. FORTUNATELY; THERE WAS NO TFC OR TERRAIN CONFLICT EXPERIENCED. REVIEWING THE EVENT; FATIGUE AND MY NOT HAVING FLOWN FOR ALMOST 3 MONTHS DEFINITELY CONTRIBUTED TO THE SITUATION; BUT; I BELIEVE THE OVERALL CAUSE WAS AN OVERALL 'COMPLACENCY' IN PERFORMING SOP DUTIES BY THE ENTIRE CREW. SEVERAL TIMES; INCLUDING DURING THE COMPLETION OF THE APCH DSCNT CHKLIST; WE ALL STATED AND CONFIRMED ALTIMETERS SET '996.' AT NO TIME DID WE ACTUALLY STATE OR CONFIRM 'ALTIMETERS SET 996 HECTOPASCALS.' NOT ONE OF US CAUGHT THIS ERROR. I HAVE OVER 10 YRS OF EXPERIENCE FLYING INTERNATIONALLY; AND THE REST OF THE CREW IS EXTREMELY EXPERIENCED IN INTL OPS; SO A LACK OF FAMILIARITY WITH INTL FLYING AND ALTIMETER PROCS IS NOT THE ISSUE. THIS EVENT SERVED TO REMIND ME OF HOW ABSOLUTELY CRITICAL IT IS TO MAINTAIN A HIGH DEGREE OF ATTN AND THOROUGHNESS TO DUTIES AND PROCS THAT CAN BECOME ROUTINE OVER TIME. SUPPLEMENTAL INFO FROM ACN 797490: THE CAPT WAS FLYING. I WAS FO. ALL OF THE CREW WERE VERY TIRED. THE ATIS INFO WAS RECEIVED OVER THE COCKPIT PRINTER. THE PRINTOUT SAID THAT THE ALTIMETER WAS 996 HP. APPARENTLY THE CAPT READ IT INCORRECTLY AS 29.96 INCHES. I HAD SET MY ALTIMETER TO 996 HP. DURING THE ARR I LOOKED AT THE CAPT'S ALTIMETERS AND SAW THAT HE HAD SET 29.96 INCHES. I INCORRECTLY DECIDED THAT I MUST HAVE SET THE WRONG ALTIMETER SETTING AND CHANGED MINE TO MATCH THE CAPT'S. NEITHER RELIEF PLT NOTICED THE ALTIMETER ERRORS. JUST OUTSIDE THE FAF; ATC QUERIED WHETHER THE ACFT WAS LOW WHICH CAUSED MOMENTARY CONFUSION. THEN ATC ASKED WHETHER WE WERE USING 996 HP OR 29.96 INCHES -- THEY STATED THAT THE CORRECT ALTIMETER WAS 996 HP AND THAT WE SEEMED ABOUT 400 FT LOW. THE CAPT TOOK SEVERAL SECONDS TO FIGURE OUT WHAT WAS WRONG AND ONE OF THE RELIEF PLTS RESTATED THAT OUR ALTIMETERS WERE SET INCORRECTLY AND THAT WE NEEDED TO CLB IMMEDIATELY. THE CAPT THEN DISENGAGED THE AUTOPLT AND QUICKLY CLBED TO THE CORRECT ALT WHILE SETTING THE ALTIMETERS TO 996 HP. ATC DID NOT MENTION THE EVENT AGAIN AND SUBSEQUENTLY CLRED US FOR THE APCH. THE ERRORS WHICH TOOK PLACE WERE SIGNIFICANTLY DUE TO MY FATIGUE AND THE FATIGUE OF THE OTHER CREW MEMBERS. THE CAPT AND I HAD JUST BEEN AWAKENED FROM A DEEP SLEEP ABOUT 1 HR AND 15 MINS EARLIER. THE RELIEF PLTS HAD BEEN AWAKE FOR APPROX 7 OR 8 HRS AND WERE BOTH UNABLE TO GET MUCH REAL SLEEP ON THEIR BREAK. THERE WERE NO TCAS NOR ANY TERRAIN PROX WARNINGS DURING THE FLT. SOME INTL ARPTS STATE ALTIMETER SETTINGS IN BOTH HP AND INCHES OF MERCURY WHICH LIKELY CONTRIBUTED TO THE CONFUSION AND ALTIMETER SETTING ERRORS. SUPPLEMENTAL INFO FROM ACN 797492: THE CAPT AND FO HAD BEEN AWAKENED AND TOOK THEIR CREW POS ABOUT 1 HR EARLIER. THE RELIEF PLTS HAD BEEN UP ABOUT 20 HRS AND BOTH ONLY MANAGED TO SLEEP ABOUT 2 HRS ON THEIR BREAK WHICH TOOK PLACE ABOUT 7 HRS EARLIER. NO TCAS OR TERRAIN PROX WARNINGS OR ALERTS OCCURRED DURING THE FLT.