Can Wal-mart get the magic back? In the four years since Wal-mart's founder died of bone cancer, CEO David Glass has invested billions of borrowed dollars in businesses that have earned relatively low returns. Glass, age 60, seemed to be a good choice for CEO when Sam Walton, at 69, tapped him to take over in 1988. Investors have knocked down Wal-Mart's market value by $7.7 billion since the day Sam died. Sam built it from nothing to $59.3 billion. In the past year, key executives, particularly Sam's favorites, have left behind Wal-Mart to pursue jobs elsewhere. The biggest shocker came at the end of March when Bill Fields, Glass's heir apparent, told his boss he wa