Caltrain No. 215 struck the bumper at about 8 a.m., Caltrain spokeswoman Christine Dunn said.
"The train was coming into the station and was going less than 5 miles per hour when it hit the bumper at the end of the platform," Talmadge said.
The four injured passengers are all women, San Francisco fire Lt. Mindy Talmadge said. Two were taken to hospitals and the other two declined to be transported.
One passenger had a tooth knocked out, but neck pain was the main complaint, Talmadge said. None of the women's injuries were life-threatening.
The passengers who were hurt were standing up to exit the train when they were knocked down by the sudden stop, Talmadge said.
The train has been taken out of service until Caltrain determines the extent of damage to it, Dunn said.
There was no damage to the station, she said.