11-4D

11-4D was a protocol droid who was in the service of Darth Plagueis in the years before the rise of the Galactic Empire. Some time before the Clone Wars Darth Plagueis was killed by his Sith apprentice Darth Sidious, who took 11-4D as his inheritance and served Sidious for many years.

Behind the scenes
Although no longer canon, 11-4D originally appeared in the legends novel Darth Plagueis, which is also by James Luceno. Various other references to the book are present throughout Tarkin, meaning that the events of the legends book could be canon.
 * Tarkin