A four-year-old boy died after afire broke out in a residential building at Ambikapatty area of south Assam on Saturday, police said.The child, identified as Raj Nath, was with a domestic help when the blaze erupted in the flat on the fourth floor of the building around 1 pm, they said.Raj Nath was already dead by the time the fire fighters reached the spot, a police officer said.The 40-year-old domestic help, recovered in a critical condition, was admitted to Silchar Medical College Hospital, he said.Investigation has been initiated to ascertain the cause of the fire, officials said.