The story
Abbreviation of follow for follow, substituting for for the number which is pronounced the same; itself clipped from follow for a follow or follow for a follow back. Compare sub4sub.
Etymology from Wiktionary, CC BY-SA 4.0.
follow4follow
Today it means: On social media services, the practice of following a user's account in exchange for said person to follow one's account back, usually with the goal of artificially inflating one's follower count. Full definition →