Think of it this way - when you tell charger its 8S, its going to be looking for something around 25v on the batt you connect. Your charger should refuse to charge a 4S2P input as 8S since the voltage is only half that, about 13v and give you an error, saying something along the line that the connected voltage is way too low.

This is how lipo capable chargers help keep us safe when charging these.