Jan 4 (Reuters) - Exxon Mobil Corp (XOM.N) said on Tuesday that its 560,500-barrel-per-day Baytown refinery in Texas continued to operate at reduced rates following a Dec. 23 fire in a unit, while four contractors injured in the incident had been discharged from the hospital. The Baytown plant"s manager also posted on Twitter that a full investigation into the cause of the incident may "take some time to conclude." A company spokesperson said the unit involved in the fire remained shut, while other units were operating at reduced rates. "The causes of the incident have not yet been determined," the spokesperson said in an emailed response, adding that the plant continued to meet "contractual commitments." The fire occurred in a hydrotreater unit at the refinery that had been shut due to a bypass line leak and the injured were contractors who had been repairing the leak, people familiar with plant operations told Reuters. read more Hydrotreaters remove sulfur and other contaminants in producing diesel, gasoline and jet fuel. A group of homeowners who said their houses near the plant were destroyed by the fire have sued Exxon. Their lawsuit asks a Texas court to restrain the company from altering, destroying or modifying evidence relevant to the explosion until the plaintiffs" counsel can conduct its own inspection. read more Exxon Mobil"s Baytown facility is home to a chemical plant, an olefins plant and the country"s fourth-biggest oil refinery.