Berkshire's initial investment in Apple got attention past year, given Buffett's usual aversion to technology companies - apart from IBM - which he considers outside his zone of competence.
Berkshire Hathaway Inc. filed a quarterly update on its US stock portfolio with the Securities and Exchange Commission on Tuesday.
Berkshire owns 57.4 million shares of Apple as of December 31, vs 15.2 million shares three months previously, according to a regulatory filing.
Buffett, 86, said last month that he'd been accelerating stock purchases at Berkshire, which he's run for more than five decades.
Berkshire increased its bets on American, United and Delta in the fourth quarter, Tuesday's filing shows. Besides the Apple (AAPL) move, Berkshire also revealed a new stake in Monsanto (MON).
But their decisions have sometimes influenced Buffett, as when Berkshire previous year paid $32.1 billion for aircraft parts maker Precision Castparts Corp, once a Combs investment.
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Berkshire's first investment with Apple received attention in 2016 given the usual aversion by Buffett for tech companies, with the exception of IBM, which he says are outside his competence zone.
Buffett surprised many people last fall by disclosing new investments in American Airlines, Delta Air Lines, United Continental and Southwest.
The new, larger stake makes Berkshire one of Apple's 10 biggest investors.
Berkshire also reported owning about 167 million shares of Sirius XM Holdings Inc., valued at about $740 million at the end of 2016.
Berkshire's dozens of subsidiaries include auto insurer Geico, Dairy Queen, Fruit of the Loom and railroad BNSF. He has said that investments of less than $1 billion are likely to be the work of Ted Weschler or Todd Combs, who each handle about $9 billion for Berkshire.
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