Agent 4 did not originate from fanon, and more information can be found on their respective wiki(s). You may read more about it on the following wiki(s):
— “Agent 4? More like Agent Snore. ” Marie
Agent 4's official art
Both (Female by default)
Agent 4 is a young Inkling who traveled to Inkopolis for unknown reasons and was recruited by Marie for the New Squidbeak Splatoon to save the Great Zapfish and defeat DJ Octavio. Along the way, they also saved Callie from DJ Octavio's influence.
Agent 4 is completely customisable by the player, but is seen in official art as having short yellow hair and red eyes. Agent 4 also wears the Hero Suit and weilds the Hero Shot, but more weapons can be unlocked.
Agent 4 is a playable character in the Hunger Games: Smash Edition!
Marie apparently finds Agent 4 to be "Pretty Adorable".
Marie apparently also thinks Agent 4 has better hygine then Agent 3.
Agent 4 is aparently older than Agent 3, but their is much debate about this.
Just like with Agent 3 and Agent 8, Agent 4's actual name is unknown, due to them being the player character.