Police identified the four as Michael James, 64, Brian Vickery, 63, Raymond White, 57, and Luke Wheaton, 16.
A fifth person was injured in the blast, and is recovering from their injuries at home, Avon and Somerset Police said.
A major incident was declared at Wessex Water’s Bristol water recycling centre off Kings Weston Lane, Avonmouth, after reports of a “large explosion”.
Emergency services were called after a chemical tank reportedly exploded.
Giles Hyder, the head of operations for the Health and Safety Executive in the southwest, said: “We send our deepest condolences to the families of those who tragically died. It is important a joint investigation is carried out.
“We will provide specialist support to what is likely to be a complex investigation under the command of the police.”
Wessex Water chief executive Colin Skellett said: “We are all absolutely devastated by what has happened. Our hearts go out to the family, friends and colleagues of those who lost their lives during the tragic event on Thursday.
“I know from the thoughts and comments I have received from so many, that this has affected the whole Wessex Water family.
“I know Avonmouth, I worked there for many years, and I know the people, some of whom have lost their lives during this terrible incident.”
The blast happened in a silo that held treated biosolids before they are recycled to land as an organic soil conditioner.
Police said when it is appropriate, investigators will speak to the survivor, and cordon at the site is likely to remain in place in the coming days as an investigation into the cause of the incident is carried out.
Superintendent Simon Brickwood said: “I'd like to extend my heartfelt sympathies to the families of those involved in yesterday's tragic incident.
“An investigation has been launched into the causes of the explosion, with the support of partner agencies and the Health and Safety Executive.
“This is likely to be ongoing for some time and we will be keeping the victims' families informed throughout.
“We appreciate the impact this incident has had on the local community and we thank those affected for their patience while our investigative work is carried out.”