The Placer County Sheriff's Department said it had been notified by a mortuary of the death, but did not yet know the identity of the victim.
Three others at the six-bed Gold Age Villa care facility in Loomis died from eating the mushrooms in what sheriff's investigators have previously characterized as an accident.
All of the victims were sickened on Nov. 8, including the caretaker who made it.
The other people who have died were identified as 86-year-old Barbara Lopes; 73-year-old Teresa Olesniewicz and 90-year-old Frank Warren Blodgett.
California's Department of Social Services is investigating the incident, though sheriff's investigators have said the caretaker who made the soup did not know that the mushrooms were poisonous.
Mushroom poisoning can often lead to liver failure.
Fall begins the season for highly sought-after wild chanterelle mushrooms in Northern California, and for the amanita species of mushroom that include what are known as "death cap" and "death angel" varieties.
Mushroom experts said that young poisonous North American amanitas found in the San Francisco Bay Area can often look like an edible version of a wild mushroom popular in Asia.
California recorded 1,700 cases of mushroom-related illnesses from 2009 to 2010, including two deaths.
(Copyright 2012 by The Associated Press. All Rights Reserved.)