Four-way stop where one vehicle arrives first
A vehicle stops at an intersection with a four-way stop and then proceeds. A crash occurs when a second vehicle enters that intersection and collides with the first vehicle.
In the diagram, Vehicle A enters an intersection with a four-way stop and collides with Vehicle B, which is already in the intersection.

Vehicle "A" = 100%
Vehicle "B" = 0%
The rules of the road
A driver at a four-way stop must yield to drivers who have already entered the intersection and proceed only when it is safe to do so. Therefore, the Vehicle A driver would be found 100% responsible.
What does it say in the Motor Vehicle Act?
The applicable sections of the Motor Vehicle Act are:
Questions?
If you have questions about your responsibility assessment, please contact your claim representative.
If you wish to dispute the assessment, find out how to file a dispute.