A Starfleet general order named in Turnabout Intruder as the only one whose violation still carries the death penalty, cited when executions are ordered aboard the Enterprise.
General Order 4 is mentioned in the last Original Series episode, Turnabout Intruder. Its content is never stated, but it is described as the single regulation for which Starfleet still allows a sentence of death.
In the episode Janice Lester, occupying Kirk's body, has Spock tried for mutiny. When McCoy and Scott are overheard discussing whether to remove her, she condemns all three officers to be executed. Chekov protests that Starfleet forbids the death penalty, and Sulu adds that the one exception, General Order 4, has not been broken by anyone aboard.
Their refusal to obey is the turning point, since it leaves Lester without a bridge crew. The line sits awkwardly beside The Menagerie, in which the only capital offence is said to be General Order 7, the ban on visiting Talos IV.