The S&P 500 rose 4.3 percent in February, its biggest gain since October, helped by earnings and a Federal Reserve that seems to have Wall Street’s back at every turn. Stocks basically returned to where they were Jan. 1. In February, investors set aside January’s volatility for three reasons, observers said. First, earnings turned out pretty good. Second, the Fed came to the market’s side with new chair Janet Yellen reaffirming plans to keep low interest rate policies in place. Lastly, the weather. Spring will come, and winter storms that kept businesses closed and consumers away from stores will fade. That pent-up demand will help the economy recover some ground.