"Four sexes" sounds click-baitier than "two morphs." It's not like they have unique procreation, other than effectively two subspecies whom cannot interbreed (they both have males and females respectively). It's interesting a species can diverge with a large mutation and split the gene pool suddenly. More intriguing is how enough of a population could grow from the first individual and thrive.

EDIT: gotta make you wonder if there were a large forest fire, volcano, human hunters or other event that almost wipe them out, and that the survivors went through a population bottleneck, perhaps picking up the mutation along the way.