WASHINGTON — The U.S. economy grew at a robust annual rate of 4.2 percent in the second quarter, the best performance in nearly four years, though economists believe growth has slowed in the current quarter partly because of a drag from trade.The performance of the gross domestic product, the country’s total output of goods and services, was unchanged from an estimate the Commerce Department made last month, the government reported Thursday.The strong GDP performance has been cited by Trump as proof that his economic program is working.“We’re doing much better than anybody thought possible,” Trump said at a Wednesday news conference.However, a big part of that growth reflected a temporary rush to ship soybeans and other U.S. exports out before penalty tariffs triggered by Trump’s get-tough trade policies took effect.Economists believe growth has slowed in the current quarter to between 3 percent and 3.5 percent, still a solid pace. While trade boosted GDP by 1.2 percentage point in the second quarter, due to a surge in exports, it is expected to trim growth by around 1 percentage point in the third quarter.Still, economists believe the country is on track for annual growth this year of around 3 percent. That would be the best performance since 2005, three years before the 2008 financial crisis pushed the country into the worst recession since the 1930s. The country is currently in the 10th year of an economic expansion, the second longest in history. But growth has averaged a lackluster 2.2 percent, making this the weakest recovery in the post-World War II period.