When Star Trek: Picard premieres later in 2019, one of the biggest Next Generation questions it could finally answer is whether B-4 ever finished his transformation into Data. The new CBS All-Access series starring Patrick Stewart as the iconic former Captain of the Starship Enterprise will be fans' first glimpse into the 24th century since Star Trek: Nemesis in 2002, the same film that introduced Data's mentally inferior android doppelganger.
Data was a core character of Star Trek: The Next Generation (arguably the second most important character after Picard himself) for its seven-season run on TV and its four movie spinoffs. Played by Brent Spiner, he was an android created by Dr. Noonien Soong along with a malevolent prior model named Lore, although the brilliant and eccentric cyberneticist also built three failed prototypes. B-4 (also played by Spiner) was one of those prototypes and not built with the sophisticated positronic brain that Lore and Data possessed. He was used as a spy by Shinzon, Praetor of the Romulan Empire and clone of Captain Jean-Luc Picard, and planted on the U.S.S. Enterprise-E. Prior to Data sacrificing himself to save his ship, he copied his memories into B-4, hoping that would help his android "brother" evolve; though B-4 couldn't assimilate Data's memories, by the end of the film, he was singing one of Data's favorite songs, hinting that the beloved character would one day re-emerge.
Star Trek: Picard will pick up the story roughly 15 years after the end of Star Trek: Nemesis and reintroduce the ex-Admiral Picard living a life markedly different from his legendary exploits as commander of the Enterprise. From the tantalizing tidbits offered in the Star Trek: Picard teaser, Jean-Luc is now retired at his family vineyard in La Barre, France after "the unimaginable" happened; that unimaginable appears to be the destruction of Romulus. While the new series will be focused on Picard and a new crew, fans are eager to know what happened to the rest of TNG's characters - including the fate of Data/B-4.
Technically, Star Trek has already provided an answer to what happened to B-4/Data in the 2009 comic book miniseries Star Trek: Countdown, which set up the first J.J. Abrams Star Trek movie. This revealed that Geordi La Forge installed Data's emotion chip to B-4's neural net, allowing his full personality to emerge. He was now Captain of the Enterprise. However, the countless ancillary Star Trek novels and comics aren't considered canon, and while Star Trek: Discovery has pulled from tie-in stories, that's no guarantee future TV or movie canon won't change it.
Will this be something Star Trek: Picard eventually resolves? Brent Spiner hasn't been linked to the show in any way; the only Next Generation alum besides Stewart involved is Jonathan Frakes, and that's only as an episode director. And if he were to appear - as B-4 or Data - Picard would have to accommodate the fact that the actor has aged (and no amount of makeup could hide that Data already looked significantly older in Star Trek: Nemesis). That said, it could be argued that, while Dr. Soong's android creations are technically supposed to be immortal, as B-4 was a prototype his exterior also ages (similar to Arnold Schwarzenegger in later Terminator films).
Given that the new Star Trek series is singularly focused on Picard's life after Starfleet, it really isn't expected for the other TNG cast members to appear in key roles. But even so, there will hopefully be a mention, at least, of what became of B-4 and Data's memories in Star Trek: Picard to solve this lingering question.
Star Trek: Picard streams on CBS All-Access and internationally on Amazon Prime in late 2019.