1. Install the Blizzard/BattleNet App
  2. Log into the App with your Blizzard Account email and password
  3. Locate Black Ops 4 on the left side of the App window
  4. Press the blue Install button to install the game 
Note: If there is no blue Install button and instead there's an ad to buy the game again, look for an X near the top right to close out the ad. The Install button should be visible now.