From left: Greg Abel, John Ternus, John Furner, and Josh D'Amaro.Getty ImagesWarren Buffett, Tim Cook, Bob Iger, and Doug McMillon are making way for a new crop of CEOs.The new bosses of Berkshire Hathaway, Apple, Disney, and Walmart are nuts-and-bolts leaders.But management gurus say that nailing the details isn't enough to succeed as a CEO.Four iconic American CEOs are making way for a new generation of business leaders.Warren Buffett, Bob Ige…