"'Gang of Four' intelligence notifications generally are oral briefings of certain particularly sensitive non-covert action intelligence activities, including principally, but not exclusively, intelligence collection programs, that the Intelligence Community typically limits to the chairmen and ranking Members of the two congressional intelligence committees. Gang of Four notifications are not based in statute but have constituted a practice generally accepted by the leadership of the intelligence committees and that is employed when the Intelligence Community believes a particular intelligence activity to be of such sensitivity that a restricted notification is warranted in order to reduce the risk of disclosure, inadvertent or otherwise. Intelligence activities viewed as being less sensitive typically are briefed to the full membership of each committee. [...] This report reviews the history of Gang of Four notification process and compares this procedure with that of the 'Gang of Eight' notification procedure. The 'Gang of Eight' procedure is statutorily based and provides that that the chairmen and ranking Members of the intelligence committee, along with the Speaker and minority leader of the House, and Senate majority and minority leaders--rather than the full membership of the intelligence committees--are to receive prior notice of particularly sensitive covert action programs, if the President determines that limited access to such programs is essential to meet extraordinary circumstances affecting vital U.S. interests."
CRS Report for Congress, R40698