>So what percentage of people at the intersection knew to turn it into a 4-way stop?

All of them :) Everyone was so confused, they were just moving a few inches at a time, with a lot of honking and gesturing for other drivers to move or stay put. Traffic was way backed up...there was just about zero chance of an accident in this case, at least. Much more effective than no lights or flashing lights in creating caution. The DOT people might want to experiment with this technique :)