AN MD82 FLT CREW EXPERIENCES A GPWS TERRAIN WARNING AFTER A GAR INITIATED BY A LOSS OF IN-TRAIL SEPARATION BEHIND A B767 AT EWR; NJ.

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ON APCH TO RWY 4R WAS TOLD TO HOLD 170 KTS TO 5 DME (4 MI FROM TOUCHDOWN ZONE); WX WAS RPTED 1400 FT OVCST; 2 1/2 MI. CONFIGURED TO FLAP 40 DEGS EARLY SO SLOW DOWN WOULD BE QUICK. ENCOUNTERED A ROLL MOVEMENT THAT DISCONNECTED AUTOPLT; CAUGHT EARLY; RE-ENGAGED AND CONTINUED APCH NORMALLY; THEN A FEW MOMENTS LATER; APCH ADVISED WE WERE IN TRAIL OF HVY B767 AHEAD. WE ALREADY KNOW THAT FROM THE ROLL MOVEMENT JUST SECONDS EARLIER. 'CONTACT TWR 118.3; CLRED TO LAND;' I THINK WAS ISSUED THEN TOLD TO SLOW AS SOON AS POSSIBLE; THEN; 'GO AROUND FOR SPACING; CLB TO 2000 FT RWY HDG.' INITIATED A SMOOTH TRANSITION FROM THE GS AND STARTED TO CLB FOR 2000 FT. JUST ARRIVING AT LEVELOFF; WAS GIVEN 3000 FT; CONTINUED CLB TO 3000 FT. THEN WAS GIVEN TURN L TO SOMETHING NEAR W. ON THAT HDG JUST STARTED TO LEVEL AT 2900 FT FOR 3000 FT; WAS GIVEN CLB TO 4000 FT. REACHED UP TO ADJUST VERT SPD AND THEN TERRAIN; TERRAIN GOES OFF.' MSA ON THE CHART IS 2900 FT AND THAT IMMEDIATELY CAME TO MIND. WE CHKED POSSIBLE ACFT UNDERNEATH; NONE. RADAR ALTIMETER DID MOVE UPWARD (VERY QUICKLY) AND IN A QUICK MOMENT RETURNED TO OUT OF SIGHT. CAPT WAS ON PA EXPLAINING THE GAR AS THE GPWS GOES OFF (TIMING IS EVERYTHING; RIGHT?) LEVELED AT 4000 FT; FLEW ANOTHER APCH AND LANDED NORMALLY. TWR ASKED FOR BASES ON SECOND APCH AND WE RPTED 800 FT. IN MY MIND; WE WERE NOT FAR ENOUGH TO THE W TO HAVE FLOWN OVER THE RISING RIDGE TERRAIN TO THE W OF NEWARK -- NOT NEAR ANY OTHER OBSTACLES. WE WERE IN LIGHT RAIN IN THE CLOUDS. BUT IT DOES MAKE YOU THINK. I GO TO TRAINING IN A FEW WKS AND THIS FLT MADE A POINT OF HIGHLIGHTING TRAINING NEEDS.