Rule of four
The 'rule of four' is a custom followed by the US Supreme Court to decide whether a particular case is worth being heard. To be considered, the case should have at least four of the nine justices in its favor.
In the legal system, jurisdiction refers to the authority of a court to hear specific type of cases and give judgments. Jurisdiction is basically divided into two types, original jurisdiction, which refers to the authority of the court to hear a case firsthand, and appellate jurisdiction, which refers to the court's authority to hear a case upon appeal. Having said that, there is a lot more to original and appellate jurisdiction than what is mentioned in these simple definitions.

Original Jurisdiction Versus Appellate Jurisdiction

In the United States, courts having original jurisdiction are called trial courts. These courts have the authority to hear the case first and come to a decision based on trial and evidence. In contrast, courts with appellate jurisdiction are courts that are empowered to review the decisions of lower courts and change the outcome if deemed necessary.

Example: If Mr. X is charged for felony, the court where his case will be heard―the district court if it is Texas or Georgia―will have original jurisdiction over the case. If Mr. X is convicted and he decides to go to the circuit court to challenge the lower court's decision, that particular court will have appellate jurisdiction.

Appellate jurisdiction should not be mistaken for a retrial, as the appellate court doesn't hear the entire case. Instead, it simply focuses on the issue of dispute. The appellate court usually looks for clear error or unquestionably erroneous judgment by a trial court, and if it finds the same, then it can reverse or modify the decision passed by the lower court. The Federal Rules of Appellate Procedure (FRAP) governs procedures in cases of appellate jurisdiction.

After reviewing the case, the appellate court will either affirm the case and uphold the decision of the lower court or remand the case and send it back to the lower court for further action. When the lower court begins the trial, it takes the appellate court's review into consideration. If the appellate court finds that the lower court has violated some law or failed to consider the laws and facts of the case, then it is entitled to reverse the lower court's decision.

Some examples of courts with original jurisdiction include the state trial courts, juvenile courts, traffic courts, federal district courts, and even the US Supreme Court, which, despite the fact that most cases heard by it fall under the appellate jurisdiction, has original jurisdiction over certain types of cases. As for examples of courts with appellate jurisdiction, these include circuit courts, state courts of appeals, state supreme courts, federal district courts, and―once again―the US Supreme Court.

US Supreme Court Jurisdiction

As per the Article III of the US Constitution and Title 28 of the US Code, the Supreme Court has original jurisdiction over cases involving two states, the federal government and state, a citizen and state, and other matters involving the interpretation of the Constitution. The Supreme Court does have the power to review the decisions passed by lower courts, but at the same time, it also has the power to decide whether the case merits hearing, which is where the 'rule of four' comes into the picture. This is done to ensure that the invaluable time of the Supreme Court is not wasted every time an aggrieved party challenges the decision of the concerned lower court.

Additionally, there exists a concept known as concurrent jurisdiction, wherein two or more courts simultaneously have jurisdiction over the case. The US Supreme Court, for instance, has original jurisdiction concurrently with lower courts in some cases.

Whether it's the original jurisdiction or appellate jurisdiction, the Supreme Court's judgment is final. So, even if one of the concerned parties is not satisfied with the verdict, there is no option to pursue the case further.