Didn't an early chapter said there were all 3 types?
The more humanoid ones with both ears, the ones with only human ears and other animal traits and the ones with only animal ears and more animal than human?
There are some questions involved with the 4 eared concept:
1 - How does their hearing work? do both set's of ears have the same level of hearing or are they different? Do they pick up different decibel levels?
2 - How does the brain function / look? do the top ears funnel down to the bottom ears before going to the brain? are there two separate sections of the brain for hearing on each side?
(I don't know neurology so 2 may be entirely stupid sounding on my part cause I don't know how brains work in the first place.)