SAN FRANCISCO — Google Inc.'s stock price dropped by more than 4 percent yesterday, accelerating a recent shift in sentiment that has caused once-ebullient investors to become more circumspect about the online search engine leader.

Barron's cast the latest pall on Google with an article outlining risks that threaten to squeeze the company's profit margins and cut its market value in half.

The gloomy scenario further dampened investors' enthusiasm for Google, whose market value has plunged by 27 percent during the past month to wipe out nearly $40 billion in shareholder wealth. Google's shares fell $16.91, or 4.7 percent, to close at $345.70 on the Nasdaq Stock Market. The shares peaked at $475.11 on Jan. 11.

Since that peak, Google has released a fourth-quarter earnings report that didn't live up to analysts' lofty expectations and alienated some of its users by launching a censored version of its search engine in China.

Those developments have contributed to an abrupt change in perception about Mountain View, Calif.-based Google, which began 2006 as a widely revered Internet icon that seemingly could do no wrong as its shares soared from their August 2004 initial public offering price of $85.

Now, Google is increasingly being viewed as a company vulnerable to stiffer competition as well as its heavy reliance on advertising revenue growth that could taper off as companies become more sophisticatedabout online marketing.

Barron's critical piece, which warned Google's shares might drop to as low as $188, highlighted how the pendulum has swung against the company.

Many risks detailed in the Barron's article also loomed as possible pitfalls during 2004 and 2005, William Blair & Co. analyst Troy Mastin reminded investors yesterday in a research note.

"While we agree that the company is facing more headwinds today than last year, most of the fundamental issues raised by Barron's are not materially different than 12 or 18 months ago," Mastin wrote.

Most other analysts echoed Mastin' Sentiments, continuing to describe the recent downturn in Google's stock as a golden opportunity for bargain hunters.

"The simple takeaway is that Google owns the best fundamentals in the Internet sector," Citigroup analyst Mark Mahaney wrote yesterday, reiterating his belief that the company's shares will bounce back to as high as $490 during the next year.