https://www.gamespot.com/articles/black-ops-4-joins-overwatch-destiny-on-blizzards-p/1100-6459025/

During the Call of Duty: Black Ops 4 reveal stream, Activision and Treyarch revealed that the PC version will live on Blizzard's Battle.net network. In addition to the franchise making its Battle.net debut, Black Ops 4 is exclusive to Blizzard's ecosystem for PC players; you won't be seeing the game on Steam.

All of Battle.net's social features will be integrated into Black Ops 4, such as friends lists, communication tools, seeing what others are playing on the platform, and party functions. This also means players will be able to communicate across games whether they're playing Overwatch or Black Ops 4, or any game for that matter. On top of living on Battle.net, Black Ops 4 is said to be specifically optimized for PC with options to run 4K resolution, 21:9 ultra widescreen resolutions, HDR, and an uncapped framerate. The game will be running on dedicated servers for online play as well.