Following a recent tweet from games writer and reviewer Marcus Sellars, there have been multiple sources confirming that Black Ops 4 will be the next Call of Duty game. The upcoming title is being developed by Treyarch, who developed previous Black Ops titles. In addition, it has been reported that the new game will have less sci-fi focused elements following the poor reception to 2016's futuristic Infinite Warfare.
The last game to be developed by Treyarch was Black Ops III, which was released in 2015, so the timing of this game seems logical, as Activision shifted the Call of Duty development cycle from Treyarch, Sledgehammer, and Infinity Ward. However despite the recent reports, upcoming Call of Duty titles aren't usually officially announced until the Spring, so it could still be a few months before we hear anymore concrete details.