By John S. McClenahen Lots of manufacturing executives and business economists can be excused for being encouraged by July's fourth consecutive monthly advance in the Conference Board's index of leading economic indicators. But David A. Rosenberg, chief North American economist at Merrill Lynch & Co., New York, cautions executives, economists and all others against getting too excited. Indeed, he contends that five months in a row of the leading indicators "would be more impressive" since that's not happened since late-1998 and early-1999. In contrast, he notes that during the past three economically below-par years, the index of leading indicators has risen for four consecutive months three times. "The number for August will be the key as to whether or not it's time to uncork the champagne and begin to raise the 2004 forecast," says Rosenberg. The Conference Board is slated to release that number on Sept. 18.