With the release of the Berkshire Hathaway 13F for the quarter ended Dec.31, we now have the answer who was the "mystery" buyer of Apple (AAPL) stock in the fourth quarter: it appears that when many others were selling, Warren Buffett was very busy buying.

In fact, he bought so much that he is now one of the top 10 holders of AAPL stock, with a disclosed position of 57.4 million shares, just behind T Rowe price, a nearly fourfold increase in his previous AAPL holdings, increasing Berkshire's Q3 stake of 15.2 million shares by 42.1 million.

At today's all time high closing price of $135, this means that Berkshire owns a whopping $7.7 billion of Apple.

Aside from AAPL, another notable change in Berkshire's holdings is American Airlines (AAL), where he becomes the second largest holder with a 8.79% stake, rising from 4.2% previously, and sending the stock higher in the after hours.

Finally, here is a breakdown of his latest 13F:

• Top new buys: LUV, SIRI
• Top exits: DE, KMI, DNOW, LEE
• Boosted stakes in AAPL, DAL, UAL, AAL, BK
• Cut stakes in WMT, VZ

Source