Sure, 4chan was there for Anonymous, which engages in some political activity. But 4chan also spawns a massive amount of graffiti, grossness, shock images, juvenelia, etc. Most people don't think of 4chan as the brave home of political protesters; they think of it as in immature dumping ground for memes and shock porn.
If the goal is really to show the importance of anonymity online, a protester from Iran or Wikileaks would be taken far more seriously. Sending somebody from 4chan will just focus attention on the many ways that anonymity is abused.