NEW YORK – After a forceful reminder that the third quarter is history, stock investors are now uneasy about the final months of the year.

Stocks posted their biggest losses in four months Friday as traders worried that consumers won’t be able to help lift the economy. Only a day earlier, investors celebrated news that the economy grew at the fastest pace in two years in the July-September period.

The Dow Jones industrial average rose 200 points Thursday only to tumble 250 on Friday.

The market’s volatility adds urgency to a flood of data this week that coul...

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