To start the quest, Arthur will need to visit Keane's saloon in Valentine. In it, he will talk to Theodore Levin, a biographer, who will tell him about the four shooters and provide the hero with photos of each of them. Morgan will also need to borrow a camera from Theodore, which will later be useful in order to take a photo of the corpse.
The location of each arrow on the map will appear after Arthur studies each image.
Black Belle
Billy Midnight
Flaco Hernandez
Emmet Granger
Before proceeding to the next stage of the mission, you will need to complete the task "Beautiful night of debauchery" in the fourth part of RDR 2. After that you can go back to the saloon in Valentine and ask about Calloway
Calloway and Theodore Levin will be found on the Grand Corrigan in Saint-Denis. After talking to the guard, Arthur learns about the ultimate goal of the mission - slim Grant.
Slim Grant
To find Grant, you first need to talk to the Sheriff in Annesburg. Then Arthur's route will be to the outlaw camp, located West of Annesburg, near the river. Morgan will find slim tied to a tree and surrounded by bandits, which he will need to get rid of.
After grant is released, He will need to be taken to a designated location northeast of Arthur's current location.