How do I find the studs on my ceiling through 4.5" of polyiso?
Never installed polyiso but if I was presented this issue this is what I would do.

I would go one sheet of polyiso at a time and find the studs on the ceiling before I put the polyiso up.  Measure the distance of the studs from the edge of where the polyiso will be stalled and then mark the polyiso with those messurements.  Use a chalk line to give a nice straight line where the studs will be on the polyiso.  Installing will be simpler this way.