City firefighters responded Sunday to a fourth arson-related fire within a five-day period. The most recent fire was at an occupied house at 210 E. Cambridge St. City Fire Department personnel responded at 1:30 a.m. Sunday while people were inside the house. "No one was injured," Inspector Guy Aumend said. "I know it is arson. In fact, it is going to be aggravated arson with three people inside. It started near the basement. I can't tell you the details. I am still investigating."