X-4 was a male Human assassin. He was one of seven men recruited for Project Omega, an initiative by the Galactic Empire to engineer the ultimate assassin. Commander Rezi Soresh was in charge of this project. Soresh erased the identities of all participants from a database system and used surgery to alter their appearances. Subsequently, he began a reconditioning process intended to transform them into obedient, emotionless soldiers by erasing their memories and feelings.
Initially, X-4 developed friendships with the other trainees: X-1, X-2, X-3, X-5, X-6, and X-7. But, as Soresh began subjecting them to electrocution, X-4, along with the others, experienced a complete transformation of his personality and mind, resulting in the suppression of all emotions. Circa 10 BBY, they were forced to fight each other until only one survived. X-4 ultimately chose suicide, using a laser whip to hang himself. Eventually, X-7 was selected as Soresh's personal assassin.
The character X-4 was featured in a flashback scene in Rebel Force: Target, the initial installment of the Star Wars: Rebel Force series for young readers, released in 2008 and authored by Alex Wheeler.