(Corrects S&P, Nasdaq percentabe drops in 4th bullet point)
* Wal-Mart offers $4 bln for South Africa's Massmart
* AirTran soars after Southwest bid
* Unilever to buy Alberto Culver for $3.7 bln
* Dow off 0.01 pct, S&P off 0.1 pct, Nasdaq off 0.1 pct (Updates to afternoon, changes quote)
By Rodrigo Campos
NEW YORK, Sept 27 (Reuters) - U.S. stocks fell on Monday following four weeks of gains, but investors believe there is still upside as one of the best months for stocks in 20 years winds down.
The market declined despite a flurry of acquisitions, which normally signal that companies are seeing value in the market.
Consumer goods group Unilever Plc plans to buy U.S. hair care group Alberto Culver Co for $3.7 billion. Alberto Culver surged 20.3 percent to $37.88, while Unilever's New York-traded stock added 1.4 percent to $28.97.
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. Wal-Mart, a Dow component, slipped 0.8 percent to $53.65.
"Companies are not going to buy assets simply because they can add short-term growth," said Jim Paulsen, chief investment officer at Wells Capital Management in Minneapolis. "There has to be expectation of long-term sizable benefit."
The Dow Jones industrial average was off 1.56 points, or 0.01 percent, at 10,858.70. The Standard & Poor's 500 Index fell 1.35 points, or 0.12 percent, to 1,147.32. The Nasdaq Composite Index lost 1.94 points, or 0.08 percent, to 2,379.27.
The S&P 500 is up 9.2 percent since the start of September as investors have welcomed signs the economy might avoid a double-dip recession. The benchmark has posted monthly gains of more than 9 percent only twice since the start of 1992.
"So far the market seems fairly resilient in the face of what would usually be a pullback after such a dramatic run," Paulsen said.
He added that if the S&P 500 holds above key technical levels breached last week, "what lies above up to 1,200 is not much in terms of overhead resistance."
"A lot of people are developing a sense this market wants to make a run at the April highs and they'd hate to miss it," Paulsen said.
AirTran Holdings Inc jumped 61.1 percent to $7.33 after Southwest Airlines Co offered to buy the regional carrier for $7.69 per share. Southwest shot up 14.4 percent to $14.05, while the Arca airline index rose 3.2 percent.
About 3.7 billion shares had traded by midday on the New York Stock Exchange, the American Stock Exchange and Nasdaq, about average so far this session.
Declining stocks outnumbered advancers on the NYSE by a ratio of about 15 to 14 , while on the Nasdaq, about 15 stocks fell for every 11 that rose. (Reporting by Rodrigo Campos; Editing by Jan Paschal)