Hong Kong’s chief executive announced plans yesterday to spend almost $4 billion over the next decade to clean up the territory’s polluted environment and make the city “a green model for Asia.” Tung Chee-hwa, giving his third annual policy address, said the city would crack down on owners of dirty, diesel-powered vehicles, add pollution-control devices to buses and trucks, clean up its harbor, and build sewage treatment plants and waste-disposal facilities. The city of 6.8 million people has been plagued by severe pollution in recent years, particularly air quality problems, and the city’s business community warned earlier this year that environmental degradation could scare business away.