* FTSEurofirst 300 index falls 0.4 percent
* Banks reverse earlier gains; Societe Generale up
* Cobham drops on U.S. contract delays
* Fed decision due at 1815 GMT
By Joanne Frearson
LONDON, Nov 3 (Reuters) - European shares slipped on Wednesday, after rising for four consecutive days, on investors' concerns that the U.S. Federal Reserve might inject less fresh stimulus into the economy than previously thought.
The banking sector which is sensitive to changes in the economic environment reversed earlier gains, with the STOXX Europe 600 Banks down 0.2 percent.
The pan-European FTSEurofirst 300 index of top shares closed down 0.4 percent at 1,088.92 points. The index has gained around 6 percent since the beginning of September on hopes that the Fed would give the economy a substantial boost.
"There is no certainty that the Fed will do what investors are looking for and no one is prepared to take big positions before the decision," Jeremy Batstone-Carr, head of equities at Charles Stanley said.
"Investors are looking for $500 billion and a lot will ride on whether or not the Fed leaves the door open for more quantitative easing, if they don't I think there is going to be an aggressive sell-off."
The Fed statement is due out at 1815 GMT and analysts predict the Fed will buy assets worth at least $500 billion in a bid to stimulate the economy, but the scope and the pace of the bond purchase is uncertain.
However, some banks rose. French bank Societe Generale climbed 1.9 percent after it said it would not need a capital increase to meet tougher industry rules and posted forecast-beating quarterly profit.
British bank Lloyds was 2.7 percent higher, after it poached Antonio Horta-Osorio, head of Santander's fast-growing British division, to be its next chief executive in a coup that dealt a blow to its Spanish rival.
Santander fell 3.5 percent.
COBHAM SLIPS
Cobham fell 9.5 percent after the aerospace electronics group said it is unlikely to deliver earnings growth in 2010 due to continued delays in the award of U.S. defence contracts and a fragile commercial market.
"There's no precise guidance but this seems like a semi-profit warning to me," one analyst who declined to be named said.
"Roughly two-thirds of Cobham's revenues come from the U.S. and they expected the delays to recover in the second half but that hasn't happened -- I can see 2010 estimates being reduced."
Norway's Statoil slipped 5.4 percent after it slashed its 2010 oil and gas production target as heavy maintenance work in the third quarter weakened earnings and cast doubt on its plans to boost output by up to 14 percent by 2012. Next shares dropped 2.2 percent after Britain's No.2 clothing retailer posted a slightly bigger-than-expected drop in third-quarter underlying sales.
Luxury goods group Richemont rose 2.6 percent after brokers Cheuvreux and Nomura published upbeat notes ahead of the group's half-year results.
Stocks also had been initially supported by news that Republicans, viewed as more pro-business by investors, pushed Democrats decisively from power in the U.S. House of Representatives and strengthened their ranks in the Senate.
Across Europe, the FTSE 100 index was down 0.2 percent, while Germany's DAX and France's CAC 40 both slipped 0.6 percent. (Reporting by Joanne Frearson; Editing by Erica Billingham)