If you are missing the original four locked characters (Doc, Hoffman, Jim, and Karlee), you do not need a DLC unlocker. You must complete (specifically the mission "The Crossing") to unlock them for your roster.
You can unlock specific DLC Cleaners like Heng, Sharice, and Dan for use in Swarm mode by completing matches. For example, ScreenRant notes that Heng is available after 5 matches and Sharice after 25. back 4 blood dlc unlocker
Click the three dots next to the game title in your Library and select Manage Add-ons to toggle the sliders. If you are missing the original four locked
If the party leader owns the DLC, everyone in the lobby can play the new maps, campaign acts, and Ridden Hives for free. For example, ScreenRant notes that Heng is available
If anyone in your party owns the DLC, the entire team can play the new expansion Acts and enter Ridden Hives together.