By John S. McClenahen The U.S. economy hummed along at something close to an annual rate of 4% during the calendar quarter just completed, figures Merrill Lynch & Co., New York. That's about one percentage point higher than the 3% the securities firm initially foresaw. Merrill now expects fourth-quarter GDP growth to be at a sub-par 2.5% annual rate, down from its earlier forecast for a 3.5% rate. For the first quarter of 2005, Merrill expects another three months at 2.5%. "Energy alone will likely trim as much as 0.5% from GDP growth during the quarter," says David A. Rosenberg, Merrill's chief North American economist.