K2-B4 is a Separatist Tactical Droid who was stationed at the Citadel on Lola Sayu, serving under its warden Osi Sobeck. It is the secondary antagonist of the three part "Citadel" trilogy of episodes in Star Wars: The Clone Wars.
K2-B4 assisted Osi Sobeck in guarding Jedi Master Even Piell, Captain Wilhuff Tarkin and their officers as their were interrogated and tortured for information on the Nexus Route.
An elite team of Jedi and clones, as well as Astromech Droid R2-D2 and a trio of reprogrammed battle droids under his command, attempted an operation to rescue him by infiltrating the Citadel via a Separatist shuttle. As they entered the atmosphere of Lola Sayu, K2-B4 scanned the ship for life forms using the scanners, however, because the Jedi and clones had encased themselves in carbonite, the scanners did not detect them. Thus, K2-B4 allowed them to enter Lola Sayu.
Following the Jedi's successful breakout of Piell and his men, K2-B4 informed Sobeck that Count Dooku was waiting to speak with him via hologram. After being forced to admit they had lost their prisoners, Sobek ordered K2-B4 to never surprise him with a transmission from Count Dooku ever again.
K2-B4 later discovered the missing supply shuttle and had R2-D2's battle droids taken to the interrogation center.
As the Jedi and clones attempted to flee, K2-B4 was placed in charge of a blockade set up to ensure that nothing would get through. However, a rescue fleet arrived and K2-B4 was unable to stop them from extracting the Jedi and clones. Once they had been rescued, the fleet headed back to Coruscant.