4chan is a group of imageboards on the interwebz. Hahahahahahaahaha. The understatement of a lifetime.
Everyone there is anonymous until doxxed. It is electronic anarchy, including animé and manga (the original content of the 2chan site it was modeled on), pr0n, alt-right and alt-left politics. Throw in organized threats and attacks for good and evil (however you want to interpret those) and you got the place in a nutshell. Regardless, having plenty of eyebleach on hand is a given when visiting the /b/ (random) board.