How to charge a 48V battery (12V battery * 4 in series connection) with a 12V charger?
The easy (and proper?) way to charge a 48 volt battery bank is to use a 48 volt charger.

If you only have a 12 volt charger, you can charge the individual 12 volt batteries one-at-a-time without rewiring anything - your charger's negative terminal should not be connected to "Ground".

You could also use four separate 12 volt chargers, each one charging one of the four 12 volt batteries making up the 48 volt bank, as long as the outputs of the chargers are not connected to each other except at the batteries.