Firefighters fought the flames for two hours before gaining control.
Crowded living conditions force early evacuation of nearly 300 neighbors.
Losses from the fire at the Berkshire-owned Lubrizol plant will be covered by one of the company's insurance firms.
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.