It appears that Call of Duty: Black Ops 4 will not have a single-player campaign, according to Polygon. In response to Polygon‘s report, Activision responded by stating: “We don’t comment on rumor and speculation. We look forward to revealing Black Ops 4 on May 17th.”
This type of answer isn't unexpected, truthfully. From Polygon‘s reporting, it seems as though one of the main factors in Treyarch's decision to drop single-player campaign was time. Sounds a bit odd, but considering that the series is developed in three-year cycles by three studios, it's possible that the launch date from an early November to October 12 for Black Ops 4 may have contributed to the decision.
Due to the time constraints, Treyarch shifted focus to beefing up the competitive multiplayer and cooperative Zombie mode. It's also possible that there may be additional cooperative modes in lieu of the single-player campaign, but this is more of an idea because of uncertainty. As Polygon points out from the Black Ops 4 teaser Activision released in March, “Forget what you know” tagline at the end of the video may signal a fresh start coming to the game.
Activision will be revealing Call of Duty: Black Ops 4 on May 17. The game will be launching on PS4, Xbox One, and PC on October 12.