The blaze erupted in recent days in the popular Bromo Tengger Semeru National Park in East Java's Probolinggo regency.
The victims were among a 12-member firefighting team who became trapped by thick smoke and rapidly spreading flames on Thursday afternoon.
Fanned by strong winds, the fire spread quickly through dry vegetation, and firefighters operating outside the danger zone received an emergency call from their colleagues.
Rescuers later found three people dead and three others suffering severe burns at an evacuation point.
One of them died from their injuries while receiving treatment, East Java police spokesman Jules Abraham Abast said.
"The incident occurred in a remote mountainous area, complicating rescue efforts," Probolinggo police chief Asmida Rizki Siregar said.
Ms Siregar - who joined emergency crews carrying victims on stretchers for about 2km because steep terrain prevented vehicles from reaching the site - said an investigation would probe whether negligence or criminal wrongdoing contributed to the tragedy.
A major blaze in the same national park in August scorched nearly nine square kilometres of grassland and vegetation.
Indonesia is experiencing its worst wildfires in a decade, which are sweeping through the forest home of critically endangered orangutans.
The intense fires, burning mainly on the islands of Borneo and Sumatra, began in July and could continue until November, aggravated by a severe El Nino weather pattern, the government has warned.
Contributing factors also include land clearance using fire, as well as water shortages that have hampered firefighters.
Toxic haze and smoke have caused a health crisis in neighbouring countries, drifting as far as the Philippines, more than 800km away.
With Reuters