The Call of Duty series has put a more significant focus on its multiplayer aspects since Modern Warfare was released in 2007. Every entry since then has placed more and more emphasis on multiplayer (some would say to the detriment of its single-player campaigns) and according to rumor Call of Duty Black Ops 4 might not have a single-player campaign at all.
According to Polygon, sources connected to Black Ops 4 development have stated that progress on the game's single-player wouldn't be complete by the time it launched. Instead, Treyarch is focusing on expanding the multiplayer and Zombies mode with an emphasis on co-op modes to make up for the lack of single-player.
Whether or not that means Call of Duty Black Ops 4 will never get a single-player campaign or wasn't explained. The way Polygon worded it makes it seem as though their source indicated that the game's single-player has at least been worked on. However, we don't know whether the development of a campaign has been halted or only postponed until after the game's release.
I'm not sure who Polygon's sources are (since they requested anonymity), but it seems to me like this rumor might be bunk. It doesn't make a lot of sense to launch without a single-player mode when the Black Ops series has traditionally had one of the more lauded storylines in the Call of Duty franchise. It also doesn't make sense that the devs would not have enough time to complete a single-player campaign, but somehow have enough time to completely alter the scope of the game and include enhanced co-op modes.
The one way I could buy this rumor is Treyarch was told to drop Call of Duty Black Ops 4 single-player development to implement a battle royale mode in its place. However, that's not what the rumor stated, and until we hear more, I'm going to assume this is something we should all swallow with a grain of salt. With all the "leakers" and rumors that float around the industry on a daily basis, there's no telling what's true and what's pretend anymore.
Call of Duty Black Ops 4 releases on October 12, 2018.