Charging 4 Li-ion cells in series
Lithium-Ion series battery packs are pretty much always charged in series.  For high discharge / fast charge applications, there are wires connected between each of the cells that are brought out to a balancing circuit that eliminates any imbalance between the cells during/after the charging cycle.  If these imbalances are not corrected, subsequent charge/discharge cycles make the imbalance worse and worse until one or more cells gets overcharged or overdischarged and are destroyed.

If you are not very familiar with Lithium-Ion batteries and safety, it is critical that you use a properly designed and purpose built Lithium-Ion charger to charge the battery.  Inexpensive ones with built-in balancing circuits are available for model aircraft/quadrotor battery charging from all the usual vendors.