Walmart is closing four stores in Chicago, cutting its footprint in half in the third-largest U.S. city after years of mounting losses. The closings mark a retreat for the nation's largest retailer, which faces tough competition in Chicago from the likes of Target, Albertsons, and Aldi. Walmart said it had tried and failed to improve its performance by building smaller stores, localizing its merchandise offerings, and investing $70 million in recent years on store upgrades.