RPTR ENCOUNTERS BOEING 757 WAKE TURB.

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WE WERE FOLLOWING A B757 AND WERE ADVISED THAT WE HAD 4 MI OR GREATER SEPARATION. ATC WANTED US TO MAINTAIN 190 KTS TO FAF AND 170 KTS TO 'MM.' WE STARTED GETTING HIT BY WAKE TURB AND IMMEDIATELY SLOWED TO 160 KIAS TO PUT MORE DISTANCE BTWN US AND THE B757. CTLR WAS TALKING AND COULD NOT ADVISE OF OUR SLOWING AIRSPD. 2 MINS LATER THE CTLR ASKED US OUR AIRSPD AND WE SAID '160' KTS. HE ADVISED US IN HIS UPSET TONE OF VOICE WE WERE TO HOLD 190 KTS AND THEN 170 KTS. WE INCREASED OUR SPD AND LANDED WITHOUT INCIDENT. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED HE HAD NOT INTERCEPTED THE ILS GS WHEN THE PLANE ROLLED ABOUT 20 DEGS WITH AN ALARMING ABRUPTNESS. AFTER RECOVERING; OUR RESPONSE WAS TO PULL THE PWR BACK AND SLOW DOWN TO GAIN MORE DISTANCE. TRIED TO ALERT THE CTLR BUT COULD NOT GET A WORD IN EDGE WISE DUE TO FREQ CONGESTION. THE YAW AND PITCH AND FLT PATH DISPLACEMENT WERE MINIMAL BUT THE RAPIDITY OF THE ROLL BRANDED THE EVENT ON OUR MEMORY. THE WHOLE EVENT ONLY LASTED 3-4 SECONDS. THE PF ONLY HAD PARTIAL CTL WITH AILERON OF THE AIRPLANE ROLL. THE RPTR STATED HE WOULD HAVE TOLD THE CTLR EARLIER HE COULD NOT ACCEPT THE ASSIGNED SPDS IF HE HAD KNOWN THEN WHAT HE KNOWS NOW. HE SUGGESTS AN FAA POLICY OF A MINIMUM SPACING CRITERION BEHIND A BOEING 757 BE AT LEAST 5 MI.