DA50 RPTR ON ILS APCH RWY 4R RECEIVES RED FLAGS FOR LOC AND GS ON VERY SHORT FINAL AND EXECUTES MISSED APCH. ATC SAYS IT SHUT DOWN ON ITS OWN. AFTER THIS ACFT APCH; ATC WAS SWITCHING APCHS TO RWY 31C.

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I WAS ON THE ILS APCH TO RWY 4R AT MDW AND RIGHT AFTER MY ARR THE ARPT WAS GOING TO BE SWITCHING ARRS TO RWY 31C. I INTERCEPTED BOTH THE LOC AND THE GS AS PLANNED AND BEGAN THE DSCNT. ALL INDICATIONS WERE NORMAL UNTIL REACHING 400 FT ABOVE MINIMUMS AT WHICH TIME BOTH THE LOC AND THE GS BECAME 'RED FLAGGED.' WE EXECUTED A MISSED APCH AND LATER USED THE ILS TO RWY 31C AND LANDED. AFTER LNDG I QUESTIONED ATC REGARDING THE STATUS OF THE ILS TO RWY 4R AND WAS TOLD IT HAD SHUT DOWN ON ITS OWN. THE REASON FOR THIS RPT IS THAT I BELIEVE THAT SINCE THEY WERE SWITCHING APCH AND RWYS; THE ILS TO RWY 4 WAS SHUT DOWN PREMATURELY AND IF THIS IS TRUE IT IS A POTENTIALLY DANGEROUS SIT. IF I HAD MISSED SEEING THE 'RED FLAGS' AND CONTINUED THE APCH THIS COULD EASILY HAVE RESULTED IN AN ACFT ACCIDENT. LET AN ACFT INSIDE THE OM FINISH THE APCH BEFORE SHUTTING IT OFF AND SWITCHING TO ANOTHER RWY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR SATED HE WAS ABOUT 1 1/2 MI ON FINAL WHEN HE DID THE MISSED APCH. RPTR SAID THAT IT WAS THE GND CTLR HE HAD TALKED WITH AFTER LNDG ABOUT THE SHUTDOWN OF THE ILS. ANALYST CONFERRED WITH PERSON AT MDW AND LEARNED THAT THERE HAVE BEEN SOME RPTED PROBS WITH THE RWY 4R ILS. SPOKESPERSON INDICATED THEY WOULD NOT TURN OFF THE RWY 4R ILS WHEN SWITCHING TO THE RWY 31C ILS.