M4 was a medical droid that operated on the Mon Calamari Star Cruiser called the Independence during the time of the Galactic Civil War.
During the initial stages of the Galactic Civil War, M4 was stationed in the medical bay of the Rebel Alliance's MC80 Home One type Star Cruiser, the Independence. This wheeled droid, equipped with a reassuring voice meant to calm patients, was tasked with conducting medical assessments on the cruiser's crew.
The droid provided treatment to Captain Keyan Farlander in 1 BBY for his persistent dreams. Detecting a problem with Farlander's disposition, this medical droid then requested assistance from its Human medical colleague. Captain Farlander, determined to remain on active duty, unknowingly employed a Force mind trick to influence the doctor to disregard M4's assessment, instead giving him sedatives and clearing him for duty.