The farmers were hit at around 2 p.m. when they were taking shelter in a canvas tent from a sudden thunderstorm, said one of the survivors, 48-year-old Ho Nghia, said.
Le Van Tuong, 42, succumbed to severe burns after the victims were admitted to Hue Central Hospital.
The survivors - Ho Nghia, his son Ho Linh, 17, and Nguyen Van Toan, 46 - are being treated at the hospital for burns, chest pain and dizziness.