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 show. Markets are waiting to take their cue from the Federal Reserve's decision on interest rates later in the day, Malcolm Freeman, CEO of Kingdom Futures, wrote in a note. Higher U.S. rates may support a rising dollar, which would limit demand for dollar-denominated commodities such as copper from buyers paying with other currencies. "Aside from this there is also the growing paranoia around the possibility of a global trade war which appears in balance to be frightening the more speculative investors away from the sector," Freeman added. FUNDAMENTALS * SHFE COPPER: The May contract ended down 1.5 percent at 50,580 yuan ($7,989.13) a tonne, its lowest close since Sept. 22. Earlier in the session, it touched a low of 50,470 yuan and is down 4.5 percent so far this month. * LME COPPER: Three-month copper on the London Metal Exchange gave up early gains to trade down for a fifth session, falling 0.5 percent to $6,728 a tonne, as of 0725 GMT. It earlier touched a fresh three-month low of $6,720. * INVENTORIES: LME copper stocks MCUSTX-TOTAL grew by a further 3,200 tonnes on Tuesday to 322,475 tonnes, bringing this month's rise to 61 percent. * COPPER: Workers at Antofagasta PLC's Los Pelambres copper mine in Chile have opted to extend a period of government mediation in an effort to reach agreement on a new labor contract, an industry group said on Tuesday. * ALUMINIUM: Global primary aluminium output excluding China dipped to 2.009 million tonnes in February from a revised 2.221 million tonnes in January, International Aluminium Institute (IAI) data showed on Tuesday. * TARIFFS: The United Arab Emirates is asking Washington to exempt it from new U.S. tariffs on aluminium and steel and believes it has a strong case, a senior government official said on Tuesday.