The story
Abbreviation of subscribe for a subscription back or a similar phrasing, otherwise analysable as sub (“a subscription”) + 4 (“abbreviation of for”) + sub (“a subscription”). Compare follow4follow.
Etymology from Wiktionary, CC BY-SA 4.0.
sub4sub
Today it means: On the video-sharing service YouTube, the practice of subscribing to a person's channel in exchange for said person mutually subscribing back to one's channel, often with the goal of artificially inflating one's subscriber count. Full definition →