"We are going through very difficult times in Statoil," Statoil Chief Executive Officer Helge Lund said in a statement. "Four of our respected and dear colleagues are no longer with us."
Another Statoil employee is listed as missing from the Jan. 16 attack.
Militants aligned with al-Qaida attacked the Ain Amenas natural gas facility in eastern Algeria. Several hostages and militants were killed with Algerian forces stormed the facility, run by BP, Statoil and state energy company Sonatrach.
BP Chief Executive Officer Bob Dudley said that "in all likelihood" four BP employees died during the Algerian attacks. He said BP would provide any details necessary to help governments involved "pursue the murderers and exact justice."
"This has been a heavy blow for BP," he said in a message to staff.
BP started natural gas production at the facility in 2006. The company said production there was targeted at around 870 million cubic feet per day, making it the largest facility of its kind in Algeria.