A watt-hour (Wh) expresses how much energy a battery stores. The formula is: Wh = (mAh ÷ 1000) × cell voltage (3.7 V) × cell count. The label "4S" means 4 cells wired in series. For example, a 4S 5000 mAh pack works out as 5 Ah × 14.8 V ≈ 74 Wh.
That single number decides air travel. A battery under 100 Wh flies freely in hand luggage, one between 100 and 160 Wh needs the airline's approval, and anything over 160 Wh is banned from the cabin altogether.