US STOCKS-Wall Street slips after 4-week rally

* Wal-Mart offers $4 bln for South Africa's Massmart

* AirTran soars after Southwest bid

* Unilever to buy Alberto Culver for $3.7 bln

* Indexes fall: Dow 0.1 pct, S&P 0.2 pct, Nasdaq 0.2 pct

* For up-to-the-minute market news see [STXNEWS/US] (Updates to open)

By Edward Krudy

NEW YORK, Sept 27 (Reuters) - U.S. stocks fell on Monday following four weeks of gains for Wall Street as fresh worries about euro zone debt offset optimism about market values after a flurry of M&A activity.

Credit agency Moody's slashed its rating on Anglo Irish Bank's [ANGIB.UL] lower-grade debt, a move that helped to drive investors to safer assets, lifting German bunds and U.S. Treasuries.

Wal-Mart Stores Inc , the world's largest retailer, offered to buy Massmart, South Africa's third largest retailer by value, for more than $4 billion as it seeks to expand on the continent. For details, see [ID:nLDE68Q05T]

Consumer goods group Unilever Plc agreed to buy Alberto Culver Co , the U.S. hair care group, for $3.7 billion in cash. Alberto Culver rose nearly 20 percent to $37.66, while Unilever's New York-traded stock added 1.2 percent to $28.89. [ID:nLDE68Q07C]

U.S. stocks have advanced over the last four weeks, with the benchmark S&P 500 index up more than 9 percent since the start of September as investors welcomed signs the economy might avoid a double-dip recession.

The Dow Jones industrial average <.DJI> dipped 10.11 points, or 0.09 percent, to 10,850.15. The Standard & Poor's 500 Index <.SPX> fell 1.67 points, or 0.15 percent, to 1,147.00. The Nasdaq Composite Index <.IXIC> dropped 3.84 points, or 0.16 percent, to 2,377.38.

"We are nearing the end of the quarter, we've had a very strong month," said Peter Jankovskis, co-chief investment officer at OakBrook Investments in Lisle, Illinois. "We may be in for a bit of consolidation here."

The S&P 500 is heading for its best month since March 2000 and its best September since 1939. September is usually one of the weakest months for stocks.

The next resistance on the S&P 500 is at around 1,173, the highest level since May 6, when it lost nearly 100 points intraday during the "flash crash."

Shares of AirTran holdings Inc rose 59 percent to $7.24 after Southwest Airlines Co offered to buy the regional carrier for $7.69 per share. Southwest was up 5 percent to $12.88. [ID:nSGE68Q0GW]

Preorders in China for Apple Inc's iPhone 4 have exceeded 200,000 units since the smartphone went on sale Saturday, distributor China Unicom said. Apple added 0.5 percent to $293.65. [ID:nTOE68Q03F] (Reporting by Edward Krudy; editing by Jeffrey Benkoe)