An emergency official said that three high school students and a teacher on a sightseeing trip were among the victims.
Mount Sinabung erupted several times during the day, spewing columns of ash about 2,000 meters into the air and spreading hot rocks and ash over a 4.5 kilometer (3 mile) radius.
There had been a reduction in the volcano's activity since August 2010 eruptions, and nearly 14,000 people had returned to their homes.