General Motors Co.’s Factory Zero Detroit-Hamtramck Assembly Center will take four weeks of downtime starting next week as it prepares to expand production there, the Detroit automaker confirmed Tuesday. The company will expand production capacity at the plant, which is in both Detroit and Hamtramck. Workers at the plant build the GMC Hummer EV pickup, which launched at the end of 2021. The plant also is preparing for production of future electric products, including the Chevrolet Silverado EV and an SUV version of the Hummer. The move to boost production capacity comes sooner than initially p…