Here's how I would scale 4chan. All static items served from s3/cloudfront. Posted images pushed to s3/cloudfront. (Or wherever filehosting is cheapest). These are all the high bandwidth items, all thats left is the text/html, which isn't that much work.

I'd argue that all of the text could be served from 1 nice box if you wanted to(multiple boxes make it more complicated but not that much more). Send the post to each box, add it to a table/indexes in memory and write the post to disk and backups for recovery purposes. Then either update and cache every page the new post affects, or mark the pages dirty and update and cache them the next time they are requested.

Done, all pages served out of memory super fast, what am I missing?

As far as bandwidth bills, most browsers observe cache settings and won't re-download what it has already downloaded. His complaint is about getting hit too hard serving the html/text not the images.