Jeff,

Simplest answer is the 4th magnet is a ‘check’ magnet, it lets the ignition know the orientation of the hub so it can determine firing order, 1-3-2 in this case if my memory is correct. However, at starting speeds I think below 400rpm, all sparks fire.
as far as prop orientation is concerned, if the engine is running properly, it shouldn’t matter unless you want to try and get it to start in cylinder 1 everytime.