It's hard to get hold of precise statistics but most (honest) attempts at quantifying it end up at around 10+% of total Reddit users. So we're potentially talking 10s of millions of MAUs (monthly active users).
Definitely significant when compared to current Lemmy numbers
I couldn't get through all the 4chan praise to be honest.
I also lament what the internet has become in the 20+ years I've been using it, but I don't agree that 4chan is a good poster child for what it used to be. I hate that there seems to be a mystique around it now like it wasn't some amoral cesspool. In fact, the reason I signed up for Reddit about 12 yrs ago was because I felt so dirty browsing /b/
I don't know what sort of crowd generally hangs out here yet, but if we do manage to bring back some of the glory of the old internet, I really hope we can revive more than nihilistic meme culture and trolling.