What is the "rule of 4" with regard to the supreme court?

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. This is done specifically to prevent a majority of the Court from controlling the Court's docket.

Explanation:

necessary and proper clause. the congress shall have power to all laws which shall be necessary and proper for carrying into execution the foregoing powers, and all other powers vested by this constitution in the government of the united states, or in any department or officer thereof.