The rule of four is a Supreme Court of the United States practice that permits four of the nine justices to grant a writ of certiorari.[1] This is done specifically to prevent a majority of the Court from controlling the Court's docket. However, in practice, it may be that a "Rule of Five" is often used at the behest of four justices (who would rather not hear the case than lose it).[1]
The rule of four is not required by the Constitution, any law, or even the Supreme Court's own published rules.[2] Rather, it is a custom that has been observed since the Court was given discretion over which appeals to hear by the Judiciary Act of 1891, Judiciary Act of 1925 and the Supreme Court Case Selections Act of 1988.