You are certainly aware that Twitter did not start as a public square either (the original idea was to share SMS messages with a group of friends). So I am not following your argument.

My argument is that there is a place online where anyone can go and express an idea and challenge ideas of other’s. That place is (among others) 4chan. It’s not some obscure personal blog, it’s a large and influential platform. Is it working as a public square? Are you using it to challenge ideas you disagree with?