Catalonia's regional health department reported earlier this week that two people had died in the outbreak in the town of Sabadell, near Barcelona.
It raised that toll to four after two people in their eighties died on Thursday. Currently 28 people have been diagnosed with the disease since September 5, it said.
Four patients have been discharged and 20 more remain in hospital.
"It cannot be ruled out that more people may turn out to be affected in the coming days as the incubation period for the disease is 10 days maximum," the department said in a statement.
Legionnaire's disease is a lung infection caused by the legionella bacteria. It is not contagious but can multiply in water and air conditioning systems.
Investigators were waiting for the results of tests carried out at sites suspected of being linked to the outbreak, the statement said.