Sounds like 4chan rolled their own server-side page cache. It probably made more sense for 4chan to do that then, back when the internet was still pretty new.

Nowadays there are a few packages that perform a similar function, like Varnish. Varnish picks up requests from the internet and where possible, it returns the response as static HTML from its own cache.

There’s also other types of caching at different stages of the request lifecycle that already reduce the gap between dynamic and static website performance.