By Victor Reklaitis and Carla Mozee, MarketWatch

NEW YORK (MarketWatch) -- U.S. stocks closed mostly lower on Friday in the slowest trading day so far this year, with the S&P 500 pulling back from Thursday's record close.

Investors appeared to shrug at speeches by Federal Reserve Chairwoman Janet Yellen and European Central Bank President Mario Draghi.

The S&P 500(SPX) slipped by 3.97 points, or 0.2%, to finish at 1,988.40, while the Dow Jones Industrial Average(DJI) shed 38.27 points, or 0.2%, to end at 17,001.22, as each index snapped a four-day winning streak. The Nasdaq Composite(RIXF) bucked the negative trend, tacking on 6.45 points, or 0.1%, to close at 4,538.55.

Stocks traded roughly flat after Yellen's speech, then slumped to session lows after fresh Ukraine-Russia reports before paring losses. The reports said NATO viewed a buildup of Russian forces near Ukraine as alarming, and the organization condemned the entry of a Russian convoy into Ukraine.

Careful comments in Jackson Hole: Yellen, at her speech at the annual gathering of central bankers in Jackson Hole, Wyo., said the economy is getting closer to the Fed's goals of full employment and stable inflation, and the debate at the central bank is "naturally shifting" to when to raise interest rates. Balancing this more hawkish tone, Yellen said the unemployment rate's decline overstates the improvement in overall labor market conditions.

"I think the most important takeaway is that as one of the most dovish members of the board, she's clearly much less dovish than she was," Bruce McCain, chief investment strategist at Key Private Bank, told MarketWatch. He said Yellen is now making "more an argument for patience rather than dovishness."

Draghi said the European Central Bank stands ready to take more unconventional action if needed, but it can't solve the euro zone's unemployment problem all by itself.

Weekly performance: The three main indexes all achieved their third weekly gain in a row. The S&P 500 and Nasdaq both advanced by 1.7% for the week, while the Dow rose 2%.

"A number of economic indicators came in fairly positive, and I think that supported the market's uptrend this week," said Carin Pai, an equity strategist at wealth manager Fiduciary Trust. She said the data included better-than-expected figures for housing starts and jobless claims, and come amid a mostly strong second-quarter earnings season that also has supported the market's three-week advance.

Tensions between Ukraine and Russia as well as in the Middle East are causes for short-term volatility, but the resulting market dips are "buying opportunities for money that has been sitting on the sidelines," she said. While the geopolitical troubles bear watching, they aren't large enough risks to change her shop's view on the economy, she said.

Low volume, and holidays next week: Composite volumes for the New York Stock Exchange and Nasdaq dropped to their lowest levels of the year for a full trading day.

Next week, holidays could help keep trading volumes at low summertime levels. Monday is a holiday in the U.K., and by the end of next week, many traders in the U.S. may be mostly focused on the Labor Day weekend.

Movers and shakers: Keurig Green Mountain Inc.(GMCR) surged 13.2% for the best gain in the S&P 500, helped by news of a new licensing deal with Kraft Foods Group Inc.(KRFT).

Salesforce.com Inc.(CRM), Ross Stores Inc.(ROST) and GameStop Corp. (GME) were also big winners in the S&P 500 as investors cheered each company's quarterly report.

(Read more about the day's big movers here http://www.marketwatch.com/story/gamestop-soars-aeropostale-sinks-foot-locker-reports-earnings-friday-2014-08-21.)

Other markets: Oil futures (CLV4) slipped and gold futures (GCZ4) edged up. In Asia, Japan's Nikkei Average broke a nine-session winning streak, and in Europe, the Stoxx 600 lost ground.

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