WASHINGTON — The U.S. economy grew at a modest 2.8 percent annual rate in the fourth quarter of last year. The Commerce Department says Americans spent more on cars and trucks, and companies built up stockpiles. Consumer spending was up from October through December of 2011, it grew at a 2 percent annual rate, that’s up modestly from the third quarter. But growth was held back by the biggest annual government spending cuts in four decades. For the full year, the economy grew just 1.7 percent, about half as much as 2010.