Red Four designated the identifier used for the fourth pilot in Red Squadron, a callsign.
Micha was assigned the Red Four callsign circa 1 BBY, and during an engagement, (battle), she deliberately crashed her X-wing into Darth Vader as a self-sacrificing maneuver aimed at killing him. While the Rebel forces initially believed Vader had perished, the Sith Lord's survival was soon confirmed when he emerged from the debris and spoke to them.
During the Battle of Yavin, John D. Branon held the Red Four designation, but he was killed while fighting.
Following the Battle of Yavin, Bela Elar, a twi'lek pilot, took on the role of Red Four and participated in a battle within the Kiax Nebula.
In the Battle of Starkiller Base, Lieutenant Bastian flew as Red Four.