I've never got the whole 4chan humor thing. I know it exists, but it ends there. There are better sites to browse for when you have -nothing else- to do. Don't see 4chan as anything else but that. A last resort for people to browse, at the most.
It depends on what part of 4chan. As for /b/, it's safe to say that there aren't many SJs there (besides ISTJ). As for the other parts, like fitness and politics and whatever (not on a computer that I can check 4chan with to reference) I'm sure that there are tons of SJs. 4chan is cool, it has something for everyone. Even if it's just random pictures that don't really mean anything.