Final answer:
George Atzerodt was the fourth person involved in the conspiracy to assassinate President Lincoln, tasked with killing Vice President Andrew Johnson but failing to do so.
Step-by-step explanation:
The fourth person who was considered to be part of the conspiracy to assassinate President Lincoln was George Atzerodt. Alongside John Wilkes Booth, Lewis Powell, and David Herold, Atzerodt was one of the key conspirators in the assassination plot. His assigned task was to kill Vice President Andrew Johnson, but Atzerodt lost his nerve and failed to carry out the attack. He, like the others involved, was apprehended and faced the consequences of the conspiracy. Atzerodt was eventually executed by hanging alongside three other co-conspirators.