Umm... errr... the 4chan Party Van is the FBI. Back in The Day, 4chan had basically only one real rule -no child pornography- and the people running the site were extremely conscientious about cooperating with law enforcement when people broke that rule. I assume they still are: it's just there there are more rules now, though most aren't the sort that will incur the wrath of the FBI when you break them.

The harassment described in the article also occurs, of course, but that's not the party van. It's more like what they'd call a raid.