CORP ACFT HAS NMAC WITH SMA WHILE ON SEQUENCE TO APCH. NO TA GIVEN OF SMA.

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MY ACFT WAS BEING VECTORED FOR SEQUENCING INTO THE DOWNWIND LEG FOR RWY 20 AT HVN. NY APCH CTL CLRED MY ACFT TO MAINTAIN 4000 FT ON A HDG OF 360 DEG(?) FOR SEQUENCING BEHIND AN ACR MLG WHICH WAS W OF THE ARPT (AT 3000 FT). BOTH CREW LOOKING FOR VISUAL SIGHTING TO THE MLG WHICH SHOULD HAVE BEEN IN OUR 9-11 O'CLOCK POS. I MADE A VISUAL SCAN TO MY R AND WAS SURPRISED TO SEE AN SMA AT MY 2 O'CLOCK POS APPROX 1/4 MI AWAY ON AN APPARENT COLLISION COURSE. HIS COURSE WAS APPROX DUE W. THE SMA WAS EXACTLY AT MY ALT OF 4000 FT. I CALLED THE TFC TO MY COPLT AND SAID SOMETHING TO THE EFFECT THAT THE SMA COULD BE A PROBLEM. AFTER APPROX 2 SECONDS OF OBSERVATION OF THE SMA I DETERMINED THAT THERE WAS NO RELATIVE MOVEMENT OF THE SMA IN OUR WINDSCREEN AND THAT OUR 2 ACFT WERE ON A COLLISION COURSE. I TOLD MY COPLT TO TELL APCH CTL WE WERE TAKING EVASIVE ACTION; AND I INITIATED A RAPID DSCNT TO PASS BELOW THE SMA. APPARENTLY THE XMISSION TO APCH CTL WAS BLOCKED MOMENTARILY BY OTHER RADIO TFC. DURING THE NEXT BREAK IN CHATTER; THE CTLR CALLED US AND TOLD US TO MAINTAIN 4000 FT; POSSIBLY SHE HAD SEEN OUR TARGET DEVIATE FROM OUR ASSIGNED ALT. WE THEN INFORMED HER THAT WE HAD TAKEN EVASIVE ACTION TO AVOID A MIDAIR COLLISION. SHE INFORMED US THAT SHE HAD NOT OBSERVED ANY OTHER TFC AND COMMENTED THAT THE UNKNOWN ACFT WAS PROBABLY NOT TRANSPONDER EQUIPPED. OUR ACFT WENT MOMENTARILY TO 3750 FT MSL FOR LESS THAN 10 SECONDS WHILE THE SMA PASSED OVERHEAD AND SLIGHTLY AFT. WE THEN CLBED BACK TO OUR ASSIGNED ALT. THERE WAS NO CONFLICT WITH THE MLG.