The Foretellers might not be appearing in Kingdom Hearts 4.
Like it or not, Union X plays a huge role in the story of Kingdom Hearts and is essential to understanding the future, something we've already seen in the reveal trailer for Kingdom Hearts 4 with the appearance of Strelitzia in Quadratum. Despite this, Nomura says that the Foretellers might not be making a big appearance in Kingdom Hearts 4.
The Foretellers are the leaders of each Union in Union X and are very important characters to the story. They made a surprise appearance at the end of Kingdom Hearts 3, where it looked like Luxu had managed to bring them back from the past in time to be key players in the new arc.
Despite this importance and tease, it seems like they won't be making an appearance in Kingdom Hearts 4, or at least won't be as prominent as fans might have thought. As reported by KH13, Nomura had an extended interview with Famitsu to celebrate the series' 20th-anniversary, where he gave a few more details on Kingdom Hearts 4.
During the interview, Nomura said that "The 5 masters aside from Luxu have completely vanished. It is unknown if they’ll make an appearance in Kingdom Hearts 4". Another translation of the interview by aitaikimochi says that Nomura says the Foretellers have been "destroyed".
The Foretellers' appearance in the ending for Kingdom Hearts 3 implied that they'd be playing a key role in the next arc of the story, but it seems as if they might not even be in Kingdom Hearts 4. It's not clear why Luxu, who we actually knew as Xigbar, would have destroyed them or why they'd vanish, but it could have something to do with the Black Box he's been carrying around.
Nomura also confirms that the Nameless Star that appeared in Kingdom Hearts 3 does originate from Quadratum, although he says that it's a "mystery" whether she'll appear in Kingdom Hearts 4. That's a yes then, and the not-so-subtle wording here makes it seem less likely that the Foretellers will play a major role in the next game.