The Original Prusa MK4 is equipped with a break-out board for the Nextruder, the LoveBoard. The LoveBoard has four status LEDs.
The main power LED indicates the presence of the needed voltage for the LoveBoard to operate, 3.3V.
The heatsink fan cools the hotend components, including the heatsink. The associated LED indicates that this is active and spinning.
The print fan directs cool air toward the model while the model is being printed. The associated LED indicates that this is active and spinning.
The hotend heater light indicates that current is flowing through the hotend heater. During a temperature raise, the light is fully on, while at a stable hot temperature, it is expected to blink.