Authorities responded to the residential complex at Baywood and San Miguel drives. Three people were taken to a local hospital in serious condition. Authorities said the victims, described as male adults, suffered burns to their arms and face.
One person was treated at the scene for minor injuries.
It was not clear whether the victims were residents of the apartment complex or maintenance workers.
One woman who lives above the apartment where the explosion occurred said she smelled a strong odor of gas and reported it to the building management about 30 minutes before the explosion.
"We called three times because the smell was unbearable. We opened windows, we opened doors, we opened everything," said Irma Marino.
Marino's family was OK, but her apartment and another apartment caught fire.
The cause of the explosion remains under investigation. When the blast happened, residents reported that their windows shattered. One person said part of their ceiling dropped down.