The 4chan party van is another name for the FBI truck that 4chan users usually see out side their house when they post or talk about Child Pornography.
Other 4chan users use it as positive term rather than a negative one. They say it will take them away to the internet where they can live happily will small children forever. This is a lie and is only mentioned by FBI agents.
Guy1: Hey guys, here's some CP!
Guy2: Dude that's not cool.
Guy1: Uh oh, the 4Chan Party Van is outside my house! They're coming in!
Chris Hansen: Why don't you take a seat?