The company identified the final victim as 24-year-old Daniel Nicklas of Beach Park, Ill. He was a quality control chemist and had been at the company for eight months, AB Specialty Silicones said in a statement.
The three other victims, all from Wisconsin, had been identified as of Tuesday.
The other victims were identified as: Byron Biehn, 53, of Union Grove, Wis., who was a second shift production supervisor, had been with the company for nine years and was one of its owners; Jeff Cummings, 57, of Kenosha, Wis., who was the third shift production supervisor and with the company for seven years; and Allen Stevens, 29, of Paddock Lake, Wis., who was the second shift chemical operator, had been with the company for two years and was an owner of the company.
"We have lost our friend and colleagues, and know that their families have lost even more: fathers, grandfathers, husbands, sons, brothers," the statement said in part.
Autopsies were performed on all three victims pulled from the rubble and toxicology tests are pending. They are trying to determine if the victims were overcome by smoke or died in the actual explosion.
Stevens was rescued from the rubble and airlifted to Loyola Medicine's Burn Center. He died at the hospital on Saturday, from burns suffered in the explosion.
"Great guy. Worked here a couple of years. Always had a smile on his face, ton of friends, hard-working. We knew he was going to go places," Craig Schilz, Stevens' former co-worker.
Search teams recovered two bodies from the scene over the weekend.
Cummings' son said he was a supervisor who ran back in to the plant to make sure his guys were out. His family called him a hero.
Biehn's family called the loss unimaginable.
A memorial service will be held on Friday for employees, friends and families at the site of the Waukegan facility, the company said. It will honor the victims and the first responders.
Officials have concluded all scene investigations, but the investigation into the event itself is still open, the fire chief said. The AB Silicones plant will reopen May 13, but will be closed periodically for employees to attend funerals over the course of the next week, the company said.
Nine people were inside the plant at the time of the explosion, which could be felt miles away and scattered debris for blocks.
"We have been in close contact with both families and our deepest condolences go out to them during this difficult time. This is truly an awful tragedy of epic proportion. We continue to work on scene, in efforts to recover the final victim. Our condolences also go out to the entire AB Specialty Silicones family. We know this has been a trying time for you all, as well," said Lake County Coroner Howard Cooper added.
WATCH: Chopper 7HD over large explosion at AB Specialty Silicones plant
Emergency crews have had to work slowly and carefully during this recovery process.
"Every time you cut a piece away, every time you pull a piece away, you have the possibility of further collapse," Waukegan Fire Marshal Steven Lenzi said.
The next big task for crews is to piece together evidence buried at the scene and determine what caused the explosion.
"We don't want to destroy any evidence that may be under there, and we need to do it slowly and methodically, safely, for all personnel involved," Lenzi said.
The Lake County Coroner's Office, Waukegan Police Department, Waukegan Fire Department and Office of the State Fire Marshal are working together on the investigation, which could take weeks to complete.