The U.S. economy shook off the effects of the stock market crash and the drought to grow a robust 3.8 percent in 1988, the best performance in four years, the government reported Friday. The increase in the gross national product, the broadest measure of economic health, was powered by a big improvement in the U.S. trade deficit and a sharp increase in business capital investment. The advance occurred even though growth slowed sharply in the final three months of the year. From October through December, the GNP grew at an lackluster annual rate of 2 percent, the slowest quarterly performance in two years. Weakness in consumer spending and business investment held back growth. The fourth-quarter GNP increase followed a 2.5 percent rise in the July-September quarter.