Yotsuba
From The LURKMORE Wiki
The Japanese name (meaning "four leaves") of not only 4chan, but a myriad of people, fictional and otherwise. The favorite around 4chan is Yotsuba Koiwai from Azuma Kiyohiko's manga Yotsubato. She is a green-haired loli with 4 short pigtails, who is used as one of 4chan's primary mascots (the other being m<3<3t).
A substantial amount of fanart of Yotsuba has been produced for 4chan's 404 error and ban-notification pages (the latter usually also including m<3<3t and/or Anonymous), with a picture being chosen at random each time one of these pages is accessed.
404 girl
An alternate name for Yotsuba, used to troll old /b/tards into outrage at the fact that people might not instantly recognize Yotsuba for who she is. As with Cockmongler being dubbed Grinman and Smiling Man, older /b/tards worry that n00bs will mistake the troll term for the real identity of a beloved meme, thus diluting /b/'s true history.
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On 29th of April 2007 some idiot (or possibly a deliberate troll) posted in /b/ a request of Rule 34 on Yotsuba, 4chan's mascot and 404 error icon, calling her "404 girl". This was obviously the result of not LURKING MOAR and was unanimously dubbed "The failest post to ever fail", particularly since 4chan's affection for Yotsuba makes her one of the few characters of who porn was always discouraged. Since then, a number of trollings using that very same request, in a copypasta manner, were seen on /b/. Since this is a relatively new meme, /b/tards that didn't LURK MOAR often fall for it and take the OP seriously. This results in often funny threads and fetches a few lulz.
