METALS-Shanghai copper falls to 6-month low as shorts increase

(adds broker comment, updates prices) BEIJING, March 21 (Reuters) - Shanghai copper prices fell for a fourth day on Wednesday, touching their lowest in almost six months amid fears of a trade war as investors waited for news on a U.S. interest rate hike. Short positions on the most traded May copper contract on the Shanghai Futures Exchange (ShFE) have risen above 100,000 lots this week and exceed long positions by more than 16,000 lots, ShFE data show. Markets are waiting to take