Earlier quoted context omitted.
4chan is a shady place, but it is definitely not a bastion of child porn. The owners do not allow anything like that.
/b/ has been an epicentre for decades.
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]
[1]: https://samharris.org/subscriber-extras/213-worst-epidemic/