CREW IN A JETSTREAM 4100 INBOUND TO IAD INSTRUCTED TO SLOW TO 170 KIAS THEN DSND TO 4000 FT. A TRAILING ACFT ABOVE THE FLT DSNDED TO WITHIN 200 FT VERTLY THEN CLBED UP TO 6000 FT.

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THIS FLT WAS SCHEDULED OUT OF BDL TO LAND AT IAD APPROX XA10. WE WERE CLRED TO FLY THE 'ROBERT FMS TRANSITION' WITH APCH CTL CTLING THE ALTS. DURING THE TRANSITION WE WERE CLRED TO 7000 FT; THEN 5000 FT; AND TOLD TO SLOW TO 190 KIAS. WE WERE BEING SEQUENCED IN BTWN ARRIVING INTL JETS WHO ALSO FLY THE TRANSITION BUT ARE NORMALLY KEPT 1000 FT ABOVE US. THE WX CONDITIONS AT THE TIME WERE LOW CUMULUS CLOUDS AND DURING THE TRANSITION WE WERE IN AND OUT OF CLOUDS. APPROX 15-20 NM NE OF THE ARPT ATC INSTRUCTED US TO 'SLOW TO 170 KIAS; THEN DSND TO 4000 FT.' AS WE WERE SLOWING; WE RECEIVED A TCASII ACTIVATION OF A TARGET ACFT DIRECTLY OVERHEAD OF US DSNDING ON TOP OF US. WE WERE IN THE CLOUDS AT THE TIME OF THE ACTIVATION. SINCE I WAS ALMOST AT 170 KIAS; I INITIATED A DSCNT TO 4000 FT. THE TARGET ACFT BEGAN TO CLB BACK UP TO APPROX 6000 FT AND SLOWLY MOVED BEHIND US STABILIZING AT 6 NM IN TRAIL. TCASII NOTED THE CLOSEST VERT SEPARATION TO BE 200 FT. THE ACFT STAYED BEHIND US FOLLOWING US INTO IAD ARPT. THE ACFT WAS IDENTED AS A FOREIGN B747. UPON LNDG; I RPTED THE INCIDENT TO THE ATC SUPVR.