Warren Buffett loaded up on Apple Inc.'s stock AAPL, -1.59% during the fourth quarter. Buffett's investment arm Berkshire Hathaway Inc. BRK.B, -0.70% disclosed in a filing that it owned 57.36 million shares of Apple as of Dec. 31, compared with a 15.23 million-share stake as of Sept. 30. The stock rose 2.5% during the fourth quarter, and 17% since then, while the Dow Jones Industrial Average DJIA, -0.46% has gained 3.8% year to date. If he still owns that stake, it would have made $1.1 billion on his investment since the start of the year. Among other moves Berkshire made during the fourth quarter, the Wal-Mart Stores stake was slashed to 1.39 million shares from 12.97 million shares. Berkshire added a new 43.20 million-share stake in Southwest Airlines Co. LUV, -4.03% , a new 8.04 million-share bet on Monsanto Co. MON, -4.25% and a new 166.64 million-share stake in Sirius XM Holdings Inc. SIRI, -1.66% .