To charge 4 series cell, i think u need to boost the voltage
Wire the cell parallel is another solution, but i wont recommend
Because what if there is a full 4.2V cell in slot 1, and you add near dead 3.2V cell in slot 2, the current will run From 4.2V cell to 3.2V cell, and WONT BE LIMITED, until the voltage of 2 cell is same