Looking for Connection Ideas for 12V Battery Pack
Remote control hobby equipment uses various connectors, my favorite is the XT-60 for the power. For balancing the cells, most seem to agree on a JST-HX connector (yours will need to be 4 pinned). You might consider using the same interfacing hardware.

The beauty of copying an existing "standard" is that you can easily use hobby grade chargers (a lot of inexpensive but excellent choices out there).