NEW YORK (Reuters) - Stocks tumbled on Thursday, extending losses for a fourth straight session, as the Federal Reserve's weak outlook for the U.S. economy and disappointing data China heightened fears about a global recession. Big banks were the top decliners, a day after Moody's lowered debt ratings for large lenders. Shares of Citigroup Inc (NYSE:C - News) and Morgan Stanley (NYSE:MS - News) hit a 52-week low at the market's open. Citigroup fell more than 4 percent to $24.25 and Morgan Stanley dipped more than 6 percent to $12.99. -snip- "A lot of people were hoping for the Fed...