When four Muslim converts from Newburgh, N.Y., were charged last year with planning to bomb Bronx synagogues and shoot down military planes at Stewart International Airport, officials described the plan as a chilling effort to commit local terrorism…Still, civil liberties lawyers say they have been troubled as information has emerged in court cases about informants who have flattered and deceived Muslim men in one community after another. Christopher Dunn, the associate legal director of the New York Civil Liberties Union, said the case before Judge McMahon could define the limits of the terrorism sting. “The Newburgh case,” Mr. Dunn said, “may be a pivotal test of just how far the government can go.”