Warren Buffett's Berkshire Hathaway has replaced Wells Fargo as its biggest investment.

Apple held the top spot in the firm's portfolio as of December 31, according to the fourth-quarter 13-F filing published last week. Major investment firms, including hedge funds, are required to disclose their long positions in stocks every quarter. Because of a time delay, however, these positions may have changed or been closed by the time the filing is public.

Buffett's annual letter to shareholders is set to be released on Saturday.  

The top-10 list below, via Bloomberg, is ranked in ascending order of the market value of Berkshire's positions at the end of the fourth quarter, and includes the gains made in that period. It also shows whether Berkshire bought more shares, sold, or did nothing. The list excludes Kraft Heinz, as this investment is accounted for with the equity method because Berkshire owns a significant controlling stake.

Southwest Airlines

Ticker: LUV

Sector: Services

Market Value: $3.12 billion (R36.4 billion)

Market-value Change: $451.34 million  (R5.3 billion)

Q4 Position Change: Unchanged

Bank of New York Mellon

Ticker: BK

Sector: Financial

Market Value: $3.28 billion (R38.3 billion)

Market-value Change: $612.53 million (R7.1 billion)

Q4 Position Change: +10.6 million shares 

Moody's Corporation

Ticker: MCO

Sector: Services

Market Value: $3.64 billion (R43.2 billion)

Market-value Change: $207.23 million (R2.4 billion)

Q4 Position Change: Unchanged

US Bancorp

Ticker: USB

Sector: Financial 

Market Value: $4.67 billion (R54.5 billion)

Market-value Change: $106.08 million (R1.2 billion)

Q4 Position Change: +2 million shares 

Phillips 66

Ticker: PSX

Sector: Basic Materials  

Market Value: $8.16 billion (R95.2 billion)

Market-value Change: $769.78 million (R8.9 billion)

Q4 Position Change: Unchanged

American Express

Ticker: AXP

Sector: Financial 

Market Value: $15.06 billion (R175 billion)

Market-value Change: $1.34 billion (R15.3 billion)

Q4 Position Change: Unchanged

Coca-Cola

Ticker: KO

Sector: Consumer Goods  

Market Value: $18.35 billion (R214 billion)

Market-value Change: $348 million (R1.4 billion)

Q4 Position Change: Unchanged

Bank of America

Ticker: BAC

Sector: Financial

Market Value: $20.04 billion (R233 billion)

Market-value Change: $2.84 billion (R33 billion)

Q4 Position Change: Unchanged

Wells Fargo

Ticker: WFC

Sector: Financial

Market Value: $27.8 billion (R327 billion)

Market-value Change: $2.20 billion (R26 billion)

Q4 Position Change: -6 million shares 

Apple

Ticker: AAPL

Sector: Consumer Goods

Market Value: $27.98 billion (R327 billion)

Market-value Change: $7.31 billion (R85 billion)

Q4 Position Change: +31.24 million shares