A spike in supplies has sent U.S. ethanol prices tumbling to four-year lows and curbed profits for an industry that had posted robust earnings for most of this year. Ethanol futures plunged 28% last month, as falling domestic demand left U.S. producers with the largest inventories in more than a year. Stockpiles in September reached the highest level since March 2013, according to the U.S. Energy Information Administration. While demand for the biofuel, along with the gasoline into which it is blended, typically declines after the summer driving season, this year’s downshift was pronounced.