The first car to arrive at a four-way stop intersection has the right-of-way to travel through the intersection first, states About.com. If two or three cars arrive at the same time, the car on the right proceeds first.
If two cars approach from opposite directions, and one of them wishes to turn, the right-of-way goes to the car that is going straight ahead, notes About.com. When a driver waiting in line to go through an intersection reaches the front of the line, he follows the same procedure as if he had just arrived at the intersection.
If there is any doubt, it is best to yield to another driver or indicate with a hand wave that it is her turn to proceed.