Those magnetos are kind of funky by today’s standards and take a bi of re-learning.  Note that the coils fire two cylinders at the  same time, one firing the 2 upper cylinders at the same time, and the other coil fires the 2 lower cylinders at the same time.  For those of you that understand current flow, note that the spark current starts at one end of the coil, goes to a spark plug, jumps the gap, goes to the cylinder block/crankcase, to the other spark plug, jumps that gap, and back to the other end of the coil to complete the circuit.  That means that the spark jumps from the center electrode of the plug to the side electrode.  As it reaches the other plug, the spark jumps from the side electrode to the center electrode.  Not anything to get all shook up about, but interesting.  However, the electrons can leave the hotter center electrode easier than they can leave the cooler side electrode.  Again, not something to spend a lot of time worrying about.  As you are pondering all this, realize that the cylinders/crankcase are not “Ground”, but rather think of them as a big fat wire.

Since your motor is running on two cylinders, it stands to reason that that coil, points, & condenser for the running 2 cylinders are working, and the problem is with the other coil, points & condenser.  OR spark plugs if installed, or gap if using an open-air spark checker.

All this to say not much different than other mags from a practical standpoint.  As always, check the points first.