After a strong January with returns of more than 5 percent, the S&P 500 struggled to gain in February. Despite a sharp, but brief setback, the index rose for the fourth consecutive month in a row, returning 1.36 percent, and finishing just short of its record close of 1,565, a level that was reached briefly in 2007. Investors were encouraged after Ben Bernanke stood behind the central bank’s low interest rate policy and after a positive revision to fourth-quarter gross domestic product from an earlier estimate showing contraction.