Here is the reason for the 3-bolt to 4-bolt rear end swap.
four bolt on top…note the four bolts on the outer flanges, vs the three on the other.
notice the 4 bolt in front. The shaft itself is about 2mm thicker, has 4 axle cup mounting bolts, and a much much beefier joint both inner and outer.
here is a closeup of the two inboard joints.
