Both also have a very undeserved reputation. I saw news reports on 8chan as if it were something similar to Stormfront. The front page was mostly video game and pornography and one had to search very deep into it's hidden circles to find racial supremacy content.
ISPs could just drop their customers' traffic to that server. This happens all the time and has happened to 4chan before.
Or they could go further and refuse to peer with any ISP that doesn't block 4chan, or lobby to have the government compel ISPs to block it, likely on think-of-the-children or think-of-the-terrorism grounds.
Self hosting definitely isn't some magic way of preventing all censorship.
My mobile internet provider actually did this for about a year after the New Zeeland shooting. Was quite annoying and I considered switching to another company because I quite enjoyed shitposting about bicycles on /t/
You won't be able to post on 4chan and probably many other anonymous communities while on vpn unless a friend of your sets it up from a residential ip.
With TOR you have to hire someone to fill captchas just to read things.
That's a decent workaround if you're technical and stubborn, but there needs to be a solution for this that's available for everyone with just a few clicks.
Even if 4chan host their servers, if they're not in someone's basement, they'd be in colocation, which can then be targeted, since I don't think 4chan own the full datacenter.
Cloudflare did drop 8chan for political pressure.
Both also have a very undeserved reputation. I saw news reports on 8chan as if it were something similar to Stormfront. The front page was mostly video game and pornography and one had to search very deep into it's hidden circles to find racial supremacy content.