If you have a 2x2 smart charger you can directly wire up to the pack, that'd be the perfect option - no need to have jumpers/switches to separate the pairs for charging.

All that electronics is usually a microcontroller running a PWM buck converter (current control) and ADC (voltage monitoring) for each set of cells. Actually there are some neat integrated charger-in-a-chip solutions you can buy now that do everything but a lot of the good chargers still do it all explicitly: bit of a combination of manufacturers selling stuff on fancy features like mAh logging and not-invented-here syndrome.