Chilling new details about the foiled Al Qaeda plot to blow up the city's busiest subways have emerged as a fourth suspect was quietly arrested in Pakistan, the Daily News has learned. The unidentified man, who helped plan the plot, is expected to be extradited to the U.S. to betried in Brooklyn Federal Court with Adis Medunjanin and Zarein Ahmedzay of Flushing, Queens, sources said. The cooperation of would-be lead bomber Najibullah Zazi has helped law enforcement officials piece together a fuller picture of the evil plan to kill innocent straphangers around the 9/11 anniversary last year.Zazi and his two...