METALS--LME copper slips for fourth day ahead of Fed meeting

(Adds analyst comment, updates prices) BEIJING, March 20 (Reuters) - London Metal Exchange (LME) copper prices fell for a fourth session on Tuesday, tracking a fall in equities as investors trimmed positions ahead of a U.S. Federal Reserve meeting starting later in the day. The Fed is widely expected to raise interest rates for the United States this week for the sixth time since December 2015 at the two-day policy meeting, the first under new Chairman Jerome Powell. "The risk-off t