This is what you need to know before the market opens:

1. Stocks gained momentum following testimony to the Senate Banking Committee from Federal Reserve Chair Janet Yellen. Yellen reiterated Tuesday that interest rate hikes would be gradual but pointed out that it would be "unwise" to wait too long to hike again.

2. Warren Buffett increased his stake in Apple (AAPL)  in a big way during the fourth quarter. Buffett's Berkshire Hathaway (BRK.A)  holding company reported owning 57.4 million shares of Apple as of Dec. 31, up from 15.2 million shares of the tech giant at the end of the third quarter.

3. Shares of asset-management firm Fortress Investment Group (FIG)  were rising 27% in premarket trading to $7.90 after Japan's SoftBank  (SFTBF)  announced a deal to buy the company for about $3.3 billion in cash.

4. Trian Partners, led by Nelson Peltz, has taken a stake amounting to 6.42 million shares in consumer products giant Procter & Gamble (PG) , according to a new regulatory filing.

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