The blast occurred underwater at the plant on Lake Suviana, 70km (43 miles) from the city of Bologna.
The affected area is located 30m (100ft) below the surface of the lake.
The mayor of the nearby town of Camugnano told Ansa news agency that a fire was believed to have broken out during maintenance work.
Marco Masinara said rescuers had trouble gaining access and that the accident had been "serious".
Three of the four who died in the disaster were aged between 35 and 73.
