For nearly four decades, Anthony Pangaro has used an understated, diplomatic style to become one of the city’s most prolific developers, building luxury projects such as the Four Seasons Hotel and the massive Ritz-Carlton complex. He cut his teeth as a public servant overseeing one of the most contentious public works projects in Boston’s recent history, the relocation of the MBTA’s Orange Line in the South End, Roxbury, and Jamaica Plain. Now he faces the most pressure-packed test of his career: Re-starting construction of a Filene’s project that has become one of the city’s biggest real estate debacles and left a massive construction crater at the heart of Downtown Crossing for more than three years. Pangaro’s firm, Millennium Partners, took over the project last week from Vornado Realty Trust of New York, pledging to build a $500 million tower on the site with residences, offices and stores.