Fire officials said the blaze was reported shortly after 5:15 a.m. at the Terrace Apartments, a four-story, 75-unit apartment building at 926 Woodside Road.
The fire was controlled shortly before 9 a.m. but crews remained putting out hotspots later today, Redwood City Fire Marshal Jim Palisi said.
Two residents were hospitalized with moderate injuries, while two others were taken to hospitals with minor injuries, Palisi said. All four, who are adults, had been released by this afternoon, he said.
He said the rescue effort was hectic but ultimately successful, with some people escaping the building on their own and others helped out by fire crews. He said some residents climbed over balconies to get out.
The most significant damage was to the fourth floor, where the blaze appears to have started, Palisi said.
The fire marshal said the building is not habitable and there is no estimate for when residents may return, if at all.
The fire caused an estimated $3.5 million in damage to the building and roughly $500,000 in damage to its contents, Palisi said.
What caused the fire remains under investigation, but Palisi said the fire's rapid spread would have been lessened if the building had been equipped with sprinklers.
"If you have sprinklers, naturally, it's going to be diminished," he said.
Palisi said the building predates a 1989 law requiring sprinklers in new residential buildings.
"It's not a retroactive law," he added.
The American Red Cross is running an evacuation center at the Red Morton Community Center at 1120 Roosevelt Ave. to assist those displaced, spokeswoman Cynthia Shaw said.
Shaw said breakfast was served to dozens of evacuated residents. Other resources have been provided at the center throughout the day.
She encouraged residents with pets to stop by the center to be connected with partner agencies that are helping pet owners.
The building is managed by Terrace Associates Inc., which has properties throughout the Peninsula and South Bay region.
This morning's fire came several months after a fatal six-alarm fire burned at an apartment building just down the road.
That fire started at about 1:45 a.m. on July 7 at the Hallmark House Apartments, a 72-unit building at 531 Woodside Road, about half a mile from the Terrace Apartments.
A 48-year-old man was killed in that blaze and about 17 others were injured.