METALS-Shanghai copper hits 6-month low as shorts increase

(Updates position data, prices) BEIJING, March 21 (Reuters) - Shanghai copper prices fell for a fourth day on Wednesday, marking their lowest close in almost six months amid fears of a trade war as investors waited for news on a U.S. interest rate hike. Short positions held by brokerages on the most traded May copper contract on the Shanghai Futures Exchange (ShFE) rose by almost 3,000 lots to 104,540 lots on Wednesday and now exceed long positions by more than 18,000 lots, ShFE data s