First person to arrive at the 4-way stop goes first. If two cars arrive at the same time, the person on the right goes first. If more than two cars arrive (there may be some rule) but mostly someone will let someone else go and everyone will just wait their turn. We have 4-way stops everywhere over here (no 'roundabouts') and many of them have cars arriving all at the same time and it's basically just a sequence. The "A" cars go, the "B" cars go, the "C" cars go, then the "D" cars, then back to the beginning...this if its busy that is.