Lots of memes come out of 4chan. I'm not sure I'd call any of them "good" in any way beyond their being amusing. (Of note: "4chan was never good" is a meme in and of itself.)

The thousand 13-year old Bayesian LW would never "build up" anything approximating rationality, I'd conjecture. It would select for rational arguments to some extent, but it would also select for creative new obscenities, threads about how to get laid, and rationalist imagefloods (whatever that would consist of) being spammed over and over. 4chan has almost 200 million posts and I can't think of any meaningful contribution it has made to human knowledge.

Don't get me wrong, it has its purpose, but I don't believe you could ever get a community with a 4chan-style atmosphere to promote any sort of epistemic virtues, largely because I think what it would take to be noticed there would almost intrinsically require some kind of major violation of those virtues.