Google, the most-used Internet search engine, unveiled a surprise plan to sell $4 billion in shares, taking advantage of a tripling in the stock price since the company’s initial public offering a year ago yesterday.

Money raised from the sale of 14.2 million shares, or 4.8% of the total stock outstanding, may be used for acquisitions, the Mountain View, Calif.-based Google said in a regulatory filing yesterday. Google said it isn’t close to making a “material” purchase.

The sale signals Google may accelerate an expansion beyond the search, mapping, and e-mail functions that have drawn users, advertisers, and investors. Google may make a purchase as big as $2 billion in China or Russia and use the rest as a “war chest” to fend off competition from rivals that include Yahoo and Microsoft, a Goldman Sachs analyst, Anthony Noto, said.

“Their thoughts are for something more expansive” than spending on hiring and opening offices, an analyst at Turner Investment Partners Incorporated, Jason Schrotberger, said. Turner owned 450,000 Google shares as of June.

Google, which first sold shares to the public at $85 a year ago, is tapping investor appetite for its stock, prompted by a surge in Internet advertising and a fivefold rise in profit. Capital spending may increase, the company said yesterday.

The shares dropped 1.8%, or $5.05, to $280.05 at 4 p.m. New York time in Nasdaq Stock Market composite trading. They earlier fell as low as $275.

Google, co-founded by Sergey Brin and Larry Page seven years ago, has made three acquisitions since going public. The company so far has acquired Dodgeball.com, a Web site that allows cell phone users to monitor friends’ locations, and Urchin Software, which develops Web advertising technology. In October, Google bought mapping search service Keyhole and on Wednesday acquired Android. Google also owns a stake in Baidu.com, China’s largest search engine.