3-way circuit to 4-way?
Do you want four switches in total, but you only have two now? I am not sure how your parallel stairways are configured exactly and how lighting in one would affect the other. It does seem unusual. N.B. Those 4-way switches aren't cheap, about $40 each. There may be a better way to solve this altogether, like two seperate 3-way switches, independant of each other, one pair for each stairway?