Despite what a lot of idiots have to say about what s4s means, it normally comes into play with myspace sites (i.e. layout/contest/html sites).
Most sites have a support code, and when asked to s4s, two sites swap codes and paste into bulletins so their friends can see the code, and hopefully they get added.
It's not just for "myspace nerds" or whatever. Some sites out there have actual meaning.
"S4S?"
"Sure. Code's on the page. Supporting now."
Its pretty muchadvertising someone else but its done through bulletins. normally people do s4s for Layout sites, Bands etc.
on YouTube s4s means
Subscribe for Subscription You subscribe to someone because they subscribed to you.
Myspace:
Band: "hey we really need more friends can you help us get more?"
Girl: "sure but only s4s"
Band: "ok cool we posted a bulletin telling all our friends to add you, please return the favor"
-1 min. later-
Girl: "awesome I posted a bulletin to all my friends telling them to add you!"
In return both users receive a benefit.
YouTube:
Guy1: "hey everyone s4s"
Guy2: "cool, i subbed you. now subscribe back! :)"
Guy1: "thanks dude and yes i subbed back"