I loved this film. It was very different from Morris' previous work like Brass Eye or The Day Today, in that those shows never really made fun of the people in it, but the public's perception of it. Just watch the Brass Eye special, "Paedogeddon" and you'll either get what I mean, or want me hanged for suggesting you watch that sick filth. But Four Lions was very much as Bob said, a film about what would happen if The Three Stooges met Al Qaeda.
While I'd say that the film's criticism is justified because of this, this is also a lot less irreverant than the likes of Nonce Sense or Carla the Elephant, so it gets away with it.