From the start of Back 4 Blood, you will only have access to Evangelo, Walker, Holly, and Mom. However, getting access to the remaining four only requires you to play through one specific mission. The mission is hard to miss if you push through one continuous run from the start of every match. However, if you are jumping into public games, you may find you’ve accidentally missed the mission in question.
The mission itself is called The Crossing and is the fourth level in act one. To reach it quickly, you can play a run from the start on survivor difficulty. Alternatively, if you know someone who already has the checkpoint at that mission, they can host a game for you. Then you only need to complete one mission and reach the cutscene to unlock all four Cleaners.
After this, you will want to leave the run to change Cleaner and give someone new a try. This mode won’t work if you are playing the game in solo offline mode, however. There is no progression in solo, and nothing is carried over. However, Turtle Rock has said that it is looking into making changes.
You now have access to Doc, who can provide free healing. Hoffman, who creates free ammo. Jim gets improved damage when stacking headshots, and Karlee can sense mutations nearby.
NME has a list of seven things to know before jumping into Back 4 Blood for more information on how to get off to a good start.