ALTDEV ALT EXCURSION.

Narrative

WHILE ON APCH CTL VECTORS TO ILS RWY 23R AT RDU; WE WERE CLRED TO DSND TO 4000 MSL ON A HDG TO INTERCEPT THE 23R LOC. WE HAD BRIEF VISUALS WITH A WDB 'HVY' 5 MI AHEAD AND HAD BEEN ADVISED OF HIS POS AND THAT WE WERE FOLLOWING A 'HVY.' THE SCATTERED RAIN SHOWERS AND CLOUDS PRECLUDED MAINTAINING A CONSTANT VISUAL WITH THE WDB AND THE ARPT. WE INTERCEPTED AND TRACKED IN ON THE LOC AT 4000 FT UNTIL THE GS INTERCEPTED OUR GLIDE PATH. I THEN WRONGLY INITIATED A DSCNT ON THE GS WHILE MAINTAINING A TFC WATCH FOR THE WDB AND THE ARPT. AT APPROX 3500 FT; THE APCH CTLR ASKED OUR ALT AND REMARKED WE SHOULD'VE BEEN AT 4000 FT. HE ASKED IF WE HAD A VISUAL ON THE ARPT (WE DID NOW) AND WE WERE CLRED FOR A QUICK 'VISUAL APCH.' ONCE ON THE GND; AFTER AN UNEVENTFUL VISUAL APCH AND LNDG; I TALKED TO THE APCH SUPVR ABOUT THE EVENT. THE CTLR WAS VECTORING A SMALL COMMUTER ACFT UNDER THE LOC TO 23L AND NEEDED US AT 4000 FT FOR TFC SEPARATION. IF WE HAD BEEN TOLD OF THIS TFC; I DOUBT I WOULD'VE DSNDED AS READILY ON THE GS OUT OF HABIT. THE COMMENT/EXPLANATION WOULD HAVE REINFORCED THE 'MAINTAIN 4000 FT' CLRNC. ALSO; AT THE POINT OF THE CTLR'S QUERY OF OUR ALT; THE ILS GS WOULD HAVE BEEN FULL DEFLECTION SCALE DOWN IF WE HAD MAINTAINED THE 4000 FT CLRNC. THIS SITUATION WAS NOT DESIRABLE; AND TOO CLOSE IN FOR A COMFORTABLE ILS APCH WITHOUT VISUALLY IDENTING THE ARPT FOR A VISUAL APCH. THIS EVENT COULD'VE BEEN PREVENTED BY MY DILIGENTLY FOLLOWING THE ALT CLRNC; AND A BRIEF EXPLANATION FROM THE CTLR WOULD HAVE REINFORCED THE ALT CLRNC.