B-4 is a fictional character in the science fiction universe of Star Trek.

B4 was first seen in the movie Star Trek: Nemesis. He is a Soong-type android (named after Dr. Noonien Soong, their creator), supposedly created as a prototype before Lore and Data. His history is somewhat suspect since he was a tool used by Shinzon of Remus to gain tactical information of the Federation from the Enterprise.

He was found disassembled on Kolarus III . The crew reassembled him and found a "redundant memory port" on the back of his neck. His "self-actualization subroutines" were underdeveloped, and he acted like an inquisitive child. All of Data's memories and programs were transferred into B-4 in the attempt to make him more "normal" but to no avail. He was later deactivated when Shinzon's plans were revealed.

He was later reactivated when he no longer posed a threat. At the end of Nemesis it became evident that the memory transfer may have been at least partially successful. B-4 may continue to evolve - other Soong-type androids such as Data had a "childhood" of sorts where their systems evolved.