The accidents were all "minor", meaning light damage and no injuries, Chris Urmson, director of Google's self-driving car program, said on Monday.
"Not once was the self-driving car the cause of the accident," Urmson wrote.
Google revealed the crashes after the Associated Press learned that the company had notified California's Department of Motor Vehicles (DMV) that its driverless cars had been involved in three crashes since September, the month it became required to report accidents as part of the permits for tests on public roads.
Consumer advocacy group Consumer Watchdog on Monday called on the DMV to make all future incidents public; currently, the reports are confidential.
"Unbelievably, Google is planning to offer its robot cars without a steering wheel, brake pedal, or accelerator, so there would be no way for a person to take control in an emergency," said John Simpson, director of the watchdog. "That plan underscores the need for the public to know the full details of all accidents."
Consumer Watchdog also wants reports to be made public when drivers are forced to take control of a car in driverless mode.
According to the AP, the DMV had received a total of four driverless car accident reports since September. Three involved Google cars and the fourth accident involved a vehicle being trialled by parts supplier Delphi Automotive.
A source told AP that two of the accident reports concern a car in self-driving mode, while in the other two the person was required to get behind wheel. However, all four accidents happened at speeds of less than 10 miles per hour.
Urmson said Google has more than 20 cars on California's roads, and they have driven 1.7 million miles between them, one million of which were in driverless mode.
During that time, Google's cars have been hit from behind seven times at lights and on freeways. They've also been "side-swiped a couple of times" and hit by a car rolling through a stop sign, according to Urmson. He also highlighted that a Google car had avoided hitting a cyclist near its headquarters in Mountain View.
"We have a detailed review process and try to learn something from each incident, even if it hasn't been our fault," he added.
Google has previously said that its driverless cars were in a "few" accidents but, again, that none were its fault.