They just updated the story, which is even sadder:

A fourth firefighter -- one who was critically injured when three others were killed -- has died. A fifth firefighter is in critical condition. A fire department spokesman said: "This is a deliberately set arson fire. An arson fire that leads to the death of anyone constitutes murder." The firefighters were trying to protect a house from the blaze when the flames engulfed their engine.
That anyone would as cruel and stupid as to deliberately set a fire in California given the conditions on the ground currently is just unfathomable.

Our prayers are with the 400 people trapped, and with the firefighters and their families.