Four members of the Billerica Fire Department have been placed on leave while the town conducts an investigation into “sexual misconduct,” officials said. “There is a great concern that alleged activities of certain employees within the fire department may compromise the town’s ability to deliver the highest and best level of public safety service to the town and its residents,” Town Manager John Curran said. “There is no evidence that the actual public safety service or calls delivered to this point have been compromised. However, it is the highest priority of the town to ensure that this kind of alleged conduct does not occur in the future.” Curran said Billerica fire Chief Thomas Conway told him about the misconduct in January.