B737-500 FAILED TO MAKE THE TURN REQUIRED BY THE WARD 4 STAR AT PXT. THEY GOT MORE THAN 4 MI OFF COURSE. THEY WERE CORRECTING WHEN ATC CALLED THEM ON THEIR MISTAKE.

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OUR FILED FLT PLAN CALLED FOR US TO FLY DIRECTLY FROM RIC VOR TO PXT VOR THEN TO JOIN THE WARD 4 ARR INTO EWR. IT WAS THE CAPT'S LEG AND ALTHOUGH HE HAD A NUMBER OF YRS EXPERIENCE IN THE B737-100 SERIES; THIS WAS HIS SECOND MONTH ON THE B737-300/500 SERIES ACFT. WE WERE PROCEEDING IN LNAV MODE DIRECTLY TO PXT. ABOUT 10 MI W OF PXT; I WENT OFF THE RADIOS TO GET THE ATIS AND CONTACT COMPANY PER OUR PROCS IN PREPARATION FOR OUR ARR. AFTER FINISHING THIS (ABOUT 1 MIN OFF THE RADIO MONITORING) I RETURNED TO MONITORING RADIO #1 AND GAVE THE CAPT THE ATIS INFO CARD. HE MENTIONED THAT WE HAD A SIMILAR CALL SIGN SIT GOING ON WITH ZDC AND HE HAD ALREADY MESSED UP 1 CALL. I TURNED MY ATTN TO THE MCP AND NOTED WE WERE NOW IN HDG MODE AND SHOULD BE IN LNAV. I MENTIONED THIS TO THE CAPT. WE HAD JUST CROSSED OVER THE PXT VOR AND SHOULD HAVE TURNED TO INTERCEPT THE AIRWAY. THE CAPT TURNED THE HDG CURSOR BACK TO THE L TO 360 DEG HDG TO REJOIN THE AIRWAY AND THE ACFT RESPONDED CORRECTLY. THIS WOULD PROVIDE AN INTERCEPT HDG AND ALLOW US TO ENGAGE THE LNAV MODE. I SELECTED THE PROGRESS PAGE 3 ON THE CDU SCREEN FOR THE FMC AND NOTED VERBALLY WE WERE NOW 4.3 NM E OF THE AIRWAYS. JUST THEN; ZDC CALLED US AND TOLD US WE WERE OFF THE AIRWAY AND TO COME L TO 010 DEGS TO INTERCEPT THE AIRWAY AND THE WARD 4 ARR (WHICH WE HAD JUST STARTED TO DO ON OUR OWN). THERE IS AN MOA AROUND PXT; BUT NO MENTION OF ITS STATUS CAME UP.