Investment guru Warren Buffett has nearly quadrupled his investment in Apple to over 57 million shares, while selling almost all of his holding in supermarket giant Walmart. Regulatory filings for his Berkshire Hathaway investment vehicle also show a new stake in Monsanto. Observers follow Berkshire's investments closely because of Mr Buffett's successful track record. Mr Buffett surprised many people last year by disclosing new investments in American Airlines, Delta Air Lines, United Continental and Southwest. Berkshire added to the investments in American and United.