New York ( TheStreet) - Warren Buffett-run Berkshire Hathaway ( BRK.A) boosted some of its biggest investments in the first quarter of 2013 while taking new positions in Chicago Bridge & Iron ( CBI) and Liberty Media ( LMCA). Berkshire's biggest trades in 2013 appear to reflect an increasing diversity in the investment conglomerate's portfolio as Buffett adds to his 'big four' stock holdings and lieutenants Todd Combs and Ted Weschler take on larger discretionary investments. In the first quarter, Berkshire increased its stake in Wells Fargo ( WFC) by 4.2% to a total holding of over 458 million shares worth about $18 billion as of the close of Wednesday trading. Berkshire also bought 6.5 million IBM ( IBM) shares, increasing its leading stake in the tech services giant to about $14 billion as of Wednesday's close. The company also added to long-time positions such as Wal-Mart ( WMT) and DirecTV ( DTV). DirecTV, thought to be an investment by Combs and Weschler, is now among Berkshire's top stock holdings. Buffett's so called 'big four' investments include Wells Fargo, Coca-Cola ( KO), IBM and American Express ( AXP). Berskhire also added new positions in the first quarter, including a stake in industrial giant Chicago Bridge & Iron and a new position in Liberty Media. Both CB&I and Liberty Media are in the midst of consolidation efforts. In 2012, CB&I cut a $3 billion deal to buy energy specialist Shaw Group, while Liberty Media recently cut a deal to buy Virgin Media, in an expansion in the U.K. Berkshire's Liberty Media $600 million-plus stake in Liberty Media represents about 5% of the company's outstanding shares and is likely the brainchild of Weschler or Combs. The company's $400-million plus position in CB&I represents about 6% of the company's outstanding shares. It's no surprise Berkshire boosted its bets on large financial sector holdings like Wells Fargo and U.S. Bancorp. At Berkshire Hathaway's annual meeting earlier in May, Buffett gave a clean bill of health to the U.S. banking system. "I consider the banking system in the United States to be stronger than at any time in the last 25 years," Buffett said at the shareholder meeting in Omaha, Nebraska.