The Bill & Melinda Gates Foundation Trust, established by billionaire investor Bill Gates, holds significant stock holdings in various companies, with the majority of its portfolio concentrated in just four stocks. These holdings, which include Microsoft, Berkshire Hathaway, Waste Management, and Canadian National Railway, are worth billions of dollars and contribute to the trust's philanthropic efforts.

Microsoft, the software giant co-founded by Gates, tops the list of the trust's stock holdings. The Gates Foundation holds over 38.2 million shares of Microsoft stock, valued at approximately $15.45 billion. Despite Gates' departure from the company, Microsoft has thrived under the leadership of current CEO Satya Nadella, who has strategically shifted the company's focus to cloud computing and artificial intelligence. Microsoft Azure, the company's cloud platform, has become the world's No. 2 cloud infrastructure provider, generating significant revenue growth.

The second-largest stake held by the Bill & Melinda Gates Foundation Trust is in Berkshire Hathaway, chaired by renowned investor Warren Buffett. The trust owns 19.9 million shares of Berkshire stock, valued at nearly $7.96 billion. Buffett, like Gates, has pledged to donate his vast wealth to charity, and his contributions to the Gates Foundation Trust amount to $36 billion.

Waste Management, a leading provider of trash and recycling collection services, also features prominently in the trust's portfolio. With approximately 35.2 million shares of Waste Management stock, worth over $7.23 billion, the trust benefits from the predictable revenue generated by the company's recurring business model.

Lastly, the trust holds a significant stake in Canadian National Railway, with 54.8 million shares worth nearly $6.97 billion. Buffett's fondness for railroads likely influenced Gates' decision to invest in Canadian National Railway. Railroads are known for their environmental-friendly and cost-effective transportation of goods, making them an appealing long-term investment.

These four major stock holdings contribute to the trust's mission of advancing philanthropic causes worldwide. The Gates Foundation Trust has disbursed $53.8 billion over the past 24 years, tackling some of the world's most pressing issues. With its diverse portfolio and regular inflows of wealth, dividends, and donations, the trust remains well-positioned to make a significant impact in various areas of need.

Please note: The figures mentioned, including stock prices and valuations, are based on the information provided in the text.