Rescue workers battling rocketing levels of sulphurous gas on Sunday found 31 people in "cardiac arrest" near the summit of 3067-metre Mount Ontake, which erupted around noon on Saturday, police and local officials said.
The term is usually applied before doctors can certify death.
Four of those found were brought down the volcano on Sunday where they were confirmed dead, broadcaster NHK reported.
"According to the police, four people out of the 31 people who had been found in cardiac arrest near the peak were confirmed dead.
All of them were men," a station anchorman told viewers.
The news appeared to confirm fears that there was little hope for those still on the volcano.
Rescue efforts were called off mid-afternoon Sunday because the environment was becoming too dangerous for emergency workers, an official at the Nagano prefectural government told AFP.
"The rescue team suspended their operation because of the increasing concentration of sulphurous gas in the area," the official said.
Japan's volcano Ontake erupts in Nagano prefecture, central Japan.
Firefighters have separately confirmed a total 30 people with injuries, including one serious case, he said, adding that the number could still change.
Some 550 soldiers, police and firefighters took part in a major operation to reach those stranded on the volcano since it erupted during a busy weekend for tourists and hikers.
A suffocating blanket of ash up to 20cm deep covered a large area of the volcano, and had forced up to 150 to seek refuge in mountaintop shelters at one point.
Local officials believe 45 to 49 people sheltered overnight in cabins on the mountain, although details remained unclear.
The mountain is popular among walkers, particularly in late September when the turning of the autumn leaves makes for dramatic scenery.
Hikers who descended from the volcano reported scenes of horror, with stones raining down and hot ashes filling the air.
Video footage shot inside a cabin, taken shortly after the eruption and shown on NHK, revealed the screams of terrified hikers as rocks thundered against the roof and walls.
A group of 25 hikers - including a schoolchild - who spent the night in a shelter, were able to climb down Sunday to reach the start of one a trail.
A middle-aged man who was among the group said they had been near the summit.
"People panicked," he told NHK, his face smudged with ashes. "Honestly, I am glad I was able to come back alive."
Emergency helicopters rescued seven people, including two who were able to wave at a Self Defence Force helicopter.
"Originally, the rescuers thought it might be difficult to go near them because ash could rise, but the conditions were better than they believed and they were able to rescue the two people," a local official told AFP.
Among the injuries were those caused by flying rocks and internal burns from inhaling volcanic fumes, the Yomiuri Shimbun said.
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