I assume 4chan does not sell your data with 3rd parties (it pays the bills from user donations and people who buy a '4chan pass', I think advertisers might've pulled out from the site long ago, given its vulgar nature) however it does appear to comply with law enforcement. See this 4chan user who was sending death threats to a Florida sheriff on 4chan, and was later arrested for this: https://www.thedailybeast.com/tyler-meyer-busted-for-threatening-sheriff-mike-chitwood-on-4chan

I believe it's possible that 4chan had given this user's IP address to law enforcement. 4chan records your IP address when you post, and only administrators can see it: https://4chan.org/faq#postanon

You can also look at the Wikipedia entry for 4chan, similar users have been arrested for making these death threats: https://en.wikipedia.org/wiki/4chan

I don't advocate for what these users do, it's harmful, and they honestly have it coming when they get arrested--but it shows that if 4chan is compelled to reveal your information, they will.

It's not possible to post under a VPN by default, they're blocked site-wide to combat spam and automated posts, even though there's already a captcha when you post? I guess they're just trying to play it extra safe. You can post under a VPN if you buy the 4chan pass (https://www.4chan.org/pass), however 4chan does not support any privacy-centric options like Monero or even cold hard physical cash, which means even if you post under a VPN, you can still be linked to your posts via payment method.

Me personally? I lurk on there every once in a while, but I've never posted, for these reasons. At least I can post under a VPN on Lemmy.