Initial reports say the men were inspecting a chimney at the Paveneer 2002 powerhouse yesterday when they were overcome.
A fifth man stayed conscious and was able to raise the alarm.
It is believed the men were inspecting a biomass powerhouse being installed at the plant.
According to Superintendent Sittinan Sankapan of the police station at Nakuan, in the Trang district of Yantakao, investigations are continuing.
The bodies of the men were taken to Songkhnagaing Hospital in Songkhla province.