The story
Abbreviation of join for a join back or a similar phrasing, otherwise analysable as join + 4 (“abbreviation of for”) + join.
Etymology from Wiktionary, CC BY-SA 4.0.
join4join
Today it means: On the social instant messaging and VoIP service Discord, the practice of joining a person's server (collection of chatrooms) in exchange for said person mutually joining one's server back, generally with the goal of artificially inflating one's server membership count. Full definition →