By John S. McClenahen The Conference Board's index of U.S. leading economic indicators edged up only 0.1% in April, slightly below expectations. However, March's increase in the leading index was revised upward to 0.8% from 0.3%, indicating, the New York-based business research group says, that the index "is still increasing at an average annual rate of 3.5% to 4%." The index of leading indicators, which previews economic performance by about six months, now stands at 115.9 (1996=100). Meanwhile, for the second consecutive week, initial claims for unemployment insurance have risen. Initial jobless claims for the week ending May 15 were 345,000, an increase of 12,000 from the previous week's revised figure of 333,000, the U.S. Labor Department reported on May 20. However, the department's four-week moving average for initial claims -- considered by many economists to be a better reflection of labor market trends because it smoothes week-to-week fluctuations -- fell by 2,750 last week to a figure of 333,500.