Yes, Black Ops 3 is still active on Xbox One. Despite being released in 2024, the game continues to have an active player base on the Xbox One platform.
Black Ops 4, however, is basically "re-skin" of Black Ops III (for multiplayer) as there are almost no differences. Black Ops 4 only had fourteen multiplayer maps at launch with four of them being remastered versions of maps from previous games.