Four Ears

In manga and anime, sometimes characters have "four ears," yotsu-mimi 四つ耳, in the sense of having "two pairs" of ears, ni-tsui 二対, that are part of their body.

Generally, this happens when the artist adds a pair of cat ears, bunny ears, etc. on top of a human character's head that already has human ears or human-like ears.

Opera オペラ, example of cat ears, pointed ears.

Character: Opera オペラ
Anime: Mairimashita! Iruma-kun 魔入りました！入間くん (Episode 10)

Various characters in Iruma-kun have "elf ears," erufu-mimi エルフ耳 and "animal ears," kemomimi ケモミミ, although the latter is probably hair... in the same way that Kujo Jotaro's hat is probably hair.

Characters with four visible ears are few, as the lower ears are typically hidden behind hair.

Explanation

Human-shaped characters, being human, should have human ears where human ears normally go: right above their jaw. It would look weird if they didn't.

However, sometimes the artist doesn't want those ugly, skinny human ears. They want the character to have cute, fluffy cat ears, for example.

Such ears look weird if they are placed where human ears normally are, they instead look best on top of the head, so the artist draws them on top of the head, and then the character has this earless spot where their human ears normally would go.

There's nothing to fill this earless spot with. It looks absolutely weird. The character is now a skin-colored cat-lizard hybrid. Basically a forceful attempt at creating a kawaii Voldemort. This simply won't do.

So, what artists typically do, is hide the earless spot with hair. They just... cover it. And you're supposed to believe that the character's cranial structure somehow makes sense just because you can't directly see the monstrosity they've created.

Don't think hard about it. Everybody does it.

Misha ミーシャ, a cat girl, being petted by Edgar エドガー.

Left: Edgar エドガー
Right: Misha ミーシャ
Anime: Hataage! Kemono Michi 旗揚!！けものみち (Episode 10, Stitch)

Senko-san 仙狐さん, example of "washing board," sentakuita 洗濯板.

Character: Senko-san 仙狐さん
Anime: Sewayaki Kitsune no Senko-san 世話やきキツネの仙狐さん (Episode 2)

Character: Ferris フェリス
Anime: Re:Zero kara Hajimeru Isekai Seikatsu Ｒｅ：ゼロから始める異世界生活 (Episode 12)

Catherine キャサリン, example of alien cat girl.

Character: Catherine キャサリン
Anime: Gintama 銀魂 (Episode 12)

All characters above have essentially the same hair style.

This strategy clearly has setbacks: the characters must have hair long enough to cover their earless spots, and the hair must kind of land in the right spot all the time every time no matter the camera angle or the shaking of the head, which is honestly pretty weird. Kinda like a hairbra for ears.

Given this, the option that makes most sense and looks most natural is just to have four ears. That doesn't sound too crazy. Some characters have four arms, others have four eyes, not too hard to imagine having four ears being valid too.

Examples

For reference, some examples of characters with four ears:

Eris エリス, a cat girl alien, having her ears and tail checked by Kinjou Manami 金武城真奈美, and Itokazu Maki 糸嘉州マキ to make sure they're real.

Left: Kinjou Manami 金武城真奈美
Center: Eris エリス
Right: Itokazu Maki 糸嘉州マキ
Anime: Asobi ni Iku yo! あそびにいくヨ！ (Episode 1)

Aoyagi Ritsuka 青柳立夏, Soubi Agatsuma 我妻草灯, example of double ears: human ears plus "cat ears," nekomimi 猫耳.

Left: Aoyagi Ritsuka 青柳立夏
Right: Soubi Agatsuma 我妻草灯
Anime: Loveless (Episode 1)

In Loveless, everyone is born with two pairs of ears, but lose their pair of cat ears when they lose their virginity.

The Undertale character Temmie has four ears, but the lower pair is normally drawn like circles so most people have absolutely no idea that they're supposed to be ears.

Some other illustrations: