I run a 4chan archive, fireden.net, I don't archive /b/ and my #1 reason I need to take down things is because of child porn, to the point that I honestly think about why i still run it anymore.
Yes, there are points in time when posters have attempted to post child pornography there, but it gets taken down swiftly and gets reported to moderators as swiftly as its posted.
Your "epicentre" of child pornography on the Internet is Facebook, not 4chan.[1]