Because Call of Duty games work on a three-year dev cycle, it was always pretty much locked in that Treyarch would be taking the reigns of 2018's game. What wasn't confirmed though, was whether or not it was going to be fourth instalment in their Black Ops series, or something new entirely.
No game in any of the different COD sub-series have ever gone past a third title, which, combined with the fact that BLOPS 3 veered heavily into sci-fi territory while WW2 was embraced so warmly because of its grounded mentality, seemingly indicated that Treyarch could drop the Black Ops title entirely with their new game.
However, numerous rumours have essentially confirmed that 2018's release will indeed be called Black Ops 4. Obviously, of any of the COD sub-franchises, Black Ops is the most beloved (arguably even more than Modern Warfare), so it makes sense Activision wouldn't be eager to drop the one title that's actually managed to survive past a couple of games.