The Associated Press claims that three of the self-driving vehicles that had prangs were from Google's fleet of 23 modified self-driving Lexus RX SUVs. The other car is reported to be one of two Delphi development vehicles.
Both companies told the news agency that their vehicles were not at fault. Anonymous sources confirmed that all the accidents were minor and occurred at speeds under 16km/h.
Google issued a statement on the matter declaring that there were "a handful of minor fender-benders, light damage, no injuries, so far caused by human error and inattention".
It's understood that at the time of the accidents, two of the cars were in self-driving mode, while the other two were being piloted by a person in the driver's seat.
Currently there are 48 autonomous test vehicles registered to drive in the state of California. Present rules require that there is a trained and accredited person sitting in the driver's seat, who is able to take control of the vehicle in an emergency or if things don't go according to plan.
According to AP, autonomous vehicles in the USA are required to have insurance plan covering up to US$5 million ($6.3 million) in damages.
At the moment in the USA, three other states (Nevada, Michigan and Florida) and one territory (Washington, DC) permit self-driving vehicles. Those three states say that they are unaware of any accidents involving self-driving cars within their jurisdictions.