All about U4
Low voltage connection
Most U4 coils have a low voltage connector with three pins: ground, battery voltage, ignition signal.
U4 category coils like U4026 have two high voltage connections. The coils are positioned directly on the spark plug in the shaft, where the coil directly connects to the spark plug. The ignition wire at the second high voltage output connects to the other spark plug operated by this dual spark coil.