Fire crews on Friday gained full control of the Hill Fire, which burned nearly 1,600 acres near Santa Margarita.
The blaze started about 3:30 p.m. Monday near Parkhill Road and Rue de Leon and quickly swept east across the area, forcing hundreds of residents to evacuate.
The fourth house was confirmed destroyed on Thursday and was a secondary residence, likely a mobile home or a trailer, said Cal Fire spokesman Chris Elms.
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Evacuation orders were lifted Tuesday night, and all closed roads have been reopened.
All crews and equipment were released Friday afternoon, after the fire was completely contained, Elms said. All outside personnel were scheduled to leave the area by Friday night.
On Saturday, San Luis Obispo-area crews will complete one more day of fire cleanup, Elms said. Firefighters will continue sporadic patrols of the area through the weekend, he said.