The MSE-4, a product of Rebaxan Columni, was a wheeled labor droid utilized by the Techno Union at their lava mines located on the planet of Mustafar. This MSE-Series model droid was deployed before the conclusion of the Clone Wars in 19 BBY, during which the Sith Lord Darth Vader terminated the Separatist leadership who were seeking refuge at a mining facility on the planet. As Vader advanced towards his targets, a group of MSE-4 units scurried ahead, leaping over a gap in the floor to escape the Sith's presence.
As a model of labor droid, the MSE-4 was created by Rebaxan Columni and belonged to their MSE-Series droid product line. The company, owned by members of the Chadra-Fan species, originally designed the MSE droid for use on Chad, the Chadra-Fan homeworld. The MSE's design was inspired by a local pet of the world known as the pleeky, with Rebaxan Columni hoping its appearance would be perceived as endearing by the galactic market. These droids, also known as mouse droids, had a range of applications limited only by the creativity of their owners.
Classified as a fifth-degree droid, the MSE-4 stood less than 0.3 meters tall and, like the MSE-6, featured a box-shaped black body mounted on four wheels for locomotion. This mouse droid, capable of jumping over floor gaps and emitting squealing sounds, was utilized by operators of Mustafar's lava mines to scout for life-support system breaches.
Before the marketing and large-scale production of the MSE-6, Rebaxan Columni initiated test runs of the MSE-4 and MSE-5 models. This ultimately led to the company's bankruptcy near the end of the Clone Wars fought between the Galactic Republic and the Separatist Alliance. In 19 BBY, Separatist leaders were accompanied by MSE-4 units while taking shelter at their mining facility headquarters on Mustafar, a dangerous mining planet under the control of the Techno Union.
Darth Sidious, aiming to conclude the Clone Wars, dispatched his newly appointed Sith apprentice, Darth Vader, to eliminate the Separatist leaders on Mustafar. As Vader proceeded down a corridor within the Separatist headquarters, five squealing MSE-4 droids scattered in response to the approaching Sith Lord, each leaping over a break in the floor and dispersing haphazardly into the facility's control room where Vader's targets were located. Shortly thereafter, Vader sealed off the room's exits and proceeded to kill the Separatist occupants.
The MSE-4 makes an appearance in the 2005 prequel trilogy movie, Star Wars: Episode III Revenge of the Sith, and it was identified in the 2006 reference book The New Essential Guide to Droids, authored by Daniel Wallace. The scene that included the MSE-4 was filmed on August 25, 2003, at Stage 2 of Fox Studios, Sydney. For this particular scene, director George Lucas requested Industrial Light & Magic (ILM) on November 18, 2004 to render a model of the mouse droid that was seen in the 1977 original trilogy film, Star Wars: Episode IV A New Hope. Lucas's intent was to utilize the fleeing droid to emulate rats deserting a sinking ship. Subsequently, Lucas tasked ILM's Rob Coleman, Roger Guyett, and John Knoll with reducing the size of the single computer-generated mouse droid they had rendered to a quarter of its original size, which led to the creation of the smaller MSE-4 droids.