The sound of the blast was described as "deafening, like thunder."
Three were overcome by smoke inhalation while the fourth suffered heat stroke.
Complaints about the frequency of flaring resulted in a government investigation last year.
Firefighters saved the administrative building and tent storage at the site.
An investigation revealed that the production area had only one door.
In-house fire brigades join with local civil defense to extinguish the blaze.
The fire damaged machinery and destroyed a 700-square-meter section of roof.
Dangerous goods stored under hazardous conditions, according to government sources.
Owners pledge to rebuild the damaged structure in Prokopyevsk.
More than 1,300 drums remained after Lubrizol fire in Rouen, France.