According to the sources Call of Duty Black Ops 4 is underdevelopment by Treyarch.
COD WWII was released last year in April so most probably The Black Ops 2 will be released around the month of May this year.
According to the reports Black Ops has stepped down from its futuristic approach to FPS action game that they earlier produced including the COD Infinite Warfare which brought some negative response by the audience. They’ll be focusing more on thee ground reality and keep it close to the feeling of current era rather than the futuristic world.
There’s also some news regarding the the release of a Switch version.