Colorado driver manual

1) 4-WAY STOP: You must yield the right-of-way to the vehicle that reached the intersection first. When more than one vehicle reaches the intersection at the same time the vehicle on the left must yield the right-of-way and allow the vehicle on the right to go first.

A lady on her cell phone honked at me when she and I reached the 4 way stop at the same time and I went first. She was on my left. I guess the rules only work when both parties know them…