how small would her brain be when she is quad-aural? Like the bottom part is already complex enough, but then there is the cat ear also.
Is the brain differ, like a front and back part for room to the cat ear? Or the canal of the cat ear don't goo deep enough and run near the side of the skull. Or also maybe they are connected to each other from each side (all right and left) so she can hear from all ear?
Also, what kind of evolution process necessary to undergo this kind of change? Most evolution just modify already existing body part to match their needs. This is just adding more part that seems pointless.
The brain? Not that much different from ours, really.. , especially when you consider the
huge amount of brain we reserve for sight. Shift 5% of that towards aural perception and you will have full 3D sound detection without sacrificing much, if anything, in the way of other things.
Other than "because magical world" the arrangement wouldn't be that space-intensive, because a large part of the size of our inner ear is for balance, not hearing. Any extra set of ears used exclusively for hearing can be easily integrated into the skull without really changing the shape of it.
As for evolution... We evolved stereoscopic/directional hearing to be able to focus our
sight ( and our mouth) on whatever is hiding in the bushes and tries to eat us/is edible. Two ears, left and right, is enough for that.
Moveable ears gives you all-around directional hearing.
4 ears, 2 stereoscopic, 2 directional..... It would allow you to focus on what's in front of you, while still scanning for surprises all around.
Here on Earth such an advantage is evidently unnecessary, since there's not even a shred of a hint that anything with our body plan from fishes on up even tried it. Everything with a spine uses the same four modified gill bows for their ears.
In a magical world filled with monsters? Or even "bog-standard" extraterrestrial worlds. Mayhaps. Life is funny that way....