After Bozo finally gets Middle Rufus into the Rebel Camp, you meet up with Garlef keeping watch and he won't let you into the door to speak to the General.
Just keep on talking to him (press A) until you have heard all four characters. The playstation guide said to click on him 5x to get the achievement but I didn't get it.
I had to keep on clicking until I heard the fourth character I was missing.
There is the general, the dying messenger, the inscrutable rogue and the female lead.