Japan confirmed its fourth case of the A(H1N1) flu virus on Sunday in a teenager who returned from a school trip to Canada with the three other Japanese who contracted the ailment, commonly known as swine flu. The patient, who was not identified, is being quarantined at a hospital near Narita Airport, the Japanese Health and Welfare Ministry said. He had been in Oakville, Ontario, since April 24 and returned Friday on a Northwest Airlines flight from Detroit. A lab at the National Institute of Infectious Diseases confirmed that he had the same strain of the disease that has killed at least 53 people worldwide. Twenty-nine countries have now officially reported 4,379 cases of swine flu, the World Health Organization said Sunday on its Web site. Mexico has reported 1,626 confirmed human cases, including 45 deaths, while the United States has reported 2,254 laboratory-confirmed human cases, including two deaths. The Japanese authorities have been tracking travelers who arrived on the same flight as the four confirmed to have the virus. The governments in China and Hong Kong said they had quarantined travelers who were on the flight. Hong Kong is keeping two people under observation who have not shown any flu symptoms. The Chinese Health Ministry said it had seven people under observation. Their conditions were not immediately known. The Center for Disease Control in Taiwan said that 16 people from Taiwan who had been on the flight had all tested negative for the virus. Japan took 49 others who had been on the plane to hospitals for further tests, and all but four of the flight's passengers who remained in Japan have been tracked down. Meanwhile, the United States confirmed its third swine flu-related death, although it did not yet show up in the W.H.O. tally for that country. In Washington State, a man in his 30s died Thursday from what appeared to be complications from swine flu, the State Department of Health said in a statement issued Saturday. The man had underlying heart conditions and viral pneumonia at the time of his death, but swine flu was considered a factor, the statement said. There was one flu-related fatality in Costa Rica in a man who had been suffering from other complicating illnesses, the Health Ministry said. Mexico continued to gradually lift a nationwide shutdown of schools, businesses, churches and soccer stadiums. But an upswing in suspected