Warren Buffett's right-hand man and business partner Charlie Munger offers this advice for successful investing: "Carefully look at what other great investors have done." Luckily for us, great investors are required to divulge changes they make to their portfolios on a quarterly basis. These SEC 13-F filings allow us to peek into the stock comings-and-goings of money pros, including multibillion-dollar hedge fund manager Ken Fisher.

Let's take a more in-depth look at a few stocks that Fisher loaded up on, according to Fisher Asset Management's most recent quarterly 13F SEC filing.

Old favorites In the fourth quarter, Fisher increased his positions in St. Jude Medical, Inc.(NYSE:STJ) and Darling International Inc.(NYSE:DAR). St. Jude recently received a warning letter from the U.S. Food and Drug Administration  regarding design practices and quality systems at a California facility. This was widely expected after the FDA released a report in the fourth quarter regarding concern over the design and testing of the company's Durata lead. This led to a stock pullback, which Fisher likely saw and took advantage of.

Revenues and earnings were both down last quarter for Darling International. The Texas-based company provides recovery and recycling services of cooking oil and bakery waste for the food industry. Darling faces lower finished product prices, which may continue to dampen earnings. But its biodiesel joint venture with Valero Energy Corporation (NYSE:VLO) shows promise.

St. Jude and Darling possess forward price-to-earnings ratios of 11 and 13, respectively. Since the P/E ratio of the S&P 500 is currently near 17, these two stocks appear undervalued. Our Motley Fool CAPS community highly rates both as 5-star (out of 5) stocks.

New additions Fisher added Digital Realty Trust, Inc. (NYSE:DLR) , Elizabeth Arden, Inc. (NASDAQ:RDEN) , and The Hain Celestial Group, Inc. (NASDAQ:HAIN) to his portfolio in the fourth quarter. Geographically diversified REIT Digital Realty Trust will likely take advantage of the explosive growth in data storage. Global data center traffic is projected to grow fourfold by 2016. The San Francisco-based REIT invests in data center properties, such as corporate IT offices, and trades at a forward price-to-earnings ratio of 14.