SYS ERROR OCCURS WHEN THERE IS A CRITICAL AIRBORNE CONFLICT BTWN AN INBOUND DSNDING A320 ON A L BASE FOR RWY 4R AND A LEVEL NWBOUND VFR CHEYENNE AT 3500 FT; 4 NM W OF EWR; NJ.

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ON RADAR VECTORS FOR RWY 4R EWR L BASE; CLRED TO DSND FROM 4000 FT TO 2000 FT WITH NO SPDS ASSIGNED. AT ABOUT 3500 FT; BEGAN TO DECELERATE. WITH NO PRIOR NOTICE; ATC INFORMED US THERE WAS A CHEYENNE TURBOPROP AT SAME ALT IN OPPOSING DIRECTION. ATC ASKED US TO IMMEDIATELY INCREASE OUR DSCNT RATE AT WHICH POINT I DID; USING A HIGHER VERT SPD RATE. WITHIN A FEW SECONDS OF THIS; WE GOT A TCASII TA SHOWING CONFLICT TFC AT SAME ALT AND CLOSING. A FEW SECONDS LATER; TCASII ISSUED A 'DSND NOW' RA WE STILL COULD NOT VISUALLY SEE CHEYENNE. I COMPLIED WITH RA. IMMEDIATELY BY DISCONNECTING AUTOPLT AND AGGRESSIVELY PUSHING DOWN ON STICK. FO VISUALLY SAW CHEYENNE DURING THE DSCNT PUSH AND SAID SEPARATION WAS POSSIBLY AS CLOSE AS 1/2 MI. CHEYENNE PASSED OFF TO OUR UPPER R RECEIVED 'CLR OF CONFLICT' AND PROCEEDED TO A NORMAL LNDG. ISN'T THIS A CLASS B AIRSPACE? SUPPLEMENTAL INFO FROM ACN 592046: ON BASE LEG WE WERE CLRED TO DSND FROM 5000 FT TO 3000 FT. DURING THE DSCNT; AT APPROX 3500 FT; ATC ADVISED OF A GA ACFT (CHEYENNE) AT 2 O'CLOCK POS; 3500 FT. WE IMMEDIATELY RECEIVED AN RA ON OUR TCASII AND THE CAPT ACCELERATED THE DSCNT RATE. THE ACFT WAS SITED APPROX 1/2 - 3/4 MI SEPARATION; SAME ALT. ACFT PASSED TO THE R. WHEN ORIGINALLY CLRED TO DSND; THERE WAS NO SPD ASSIGNED OR TFC MENTIONED. FROM THE TIME ATC MENTIONED THE TFC TO THE TIME OF THE RA WAS ONLY 2-3 SECONDS.