For the first time, some truck-based sport utility vehicles received as many as four out of five stars in rollover ratings, according to 2004 model ratings released yesterday by federal regulators. But the higher ratings may not necessarily mean the vehicles have become safer, because the government has changed the way it tests them.
          In the new tests, using real vehicles on a test track as well as mathematical calculations, the four-wheel-drive versions of several similar sport utility vehicles made by General Motors - the Chevrolet TrailBlazer, GMC Envoy, Oldsmobile Bravada and the new Buick Rainier - earned four stars, even though they are virtually identical to G.M. vehicles that rated only three stars last year.
          Vehicle rollovers kill more than 10,000 Americans every year. They are rare but deadly, occurring in fewer than 3 percent of accidents but accounting for a third of vehicle occupant deaths. And the problem has been on the rise as sales of S.U.V.'s have boomed; sport utilities have higher centers of gravity than cars and are three times as likely to be involved in rollover deaths.