Rule of four
The rule of four states that before a case is put is accepted by the U.S. Supreme Court for resolution n the merits, four out of the nine Supreme Court justices must agree in conference to hear the merits of the case. Notice that this is less than a majority.
Justice John Harlan explained his view that five Justices should not dismiss a petition for certiorari as having been improvidently granted unless there is an intervening circumstance justifying it:
|“||In the case of a petition for certiorari that right, it seems to me -- again without the presence of intervening factors -- is exhausted once the petition has been granted and the cause set for argument. Otherwise the "rule of four" surely becomes a meaningless thing in more than one respect. First, notwithstanding the "rule of four," five objecting Justices could undo the grant by voting, after the case has been heard, to dismiss the writ as improvidently granted -- a course which would hardly be fair to litigants who have expended time, effort, and money on the assumption that their cases would be heard and decided on the merits. While in the nature of things litigants must assume the risk of "improvidently granted" dismissals because of factors not fully apprehended when the petition for certiorari was under consideration, short of that it seems to me that the Court would stultify its own rule if it were permissible for a writ of certiorari to be annulled by the later vote of five objecting Justices. Indeed, if that were proper, it would be preferable to have the vote of annulment come into play the moment after the petition for certiorari has been granted, since then at least the litigants would be spared useless effort in briefing and preparing for the argument of their cases. Second, permitting the grant of a writ to be thus undone would undermine the whole philosophy of the "rule of four," which is that any case warranting consideration in the opinion of such a substantial minority of the Court will be taken and disposed of. It appears to me that such a practice would accomplish just the contrary of what representatives of this Court stated to Congress as to the "rule of four" at the time the Court's certiorari jurisdiction was enlarged by the Judiciary Act of 1925. In effect the "rule of four" would, by indirection, become a "rule of five." Third, such a practice would, in my opinion, be inconsistent with the long-standing and desirable custom of not announcing the Conference vote on petitions for certiorari. For in the absence of the intervening circumstances which may cause a Justice to vote to dismiss a writ as improvidently granted, such a disposition of the case on his part is almost bound to be taken as reflecting his original Conference vote on the petition. And if such a practice is permissible, then by the same token I do not see how those who voted in favor of the petition can reasonably be expected to refrain from announcing their Conference votes at the time the petition is acted on.||”|
Ferguson v. Moore-McCormack, 352 U.S. 521, 559-61 (1957) (Harlan, J., concurring in one case and dissenting in three others)