Can I charge 4 12v batteries with a single12v charger?
Your electric bike probably uses a 48V power source to operate its motor.  Since you have 4 batteries, I'm assuming that your batteries are lead-acid.  Why they are soldered together I have no clue because most often such batteries are connected in series with jumper cables.  Assuming you have LA batteries that means you probably have 4 12V batteries that are connected in series--which = 48V (4 batts * 12V).

Before doing anything that I suggest below, make sure your batteries are 12V lead-acid batteries.  If they are some other chemistry (e.g. li-ion batteries), do NOT follow the advice below.

To use the charger that you bought to charge your batteries (again, assuming they are LA batteries), you should borrow/buy a soldering iron &amp; a multimeter.  Use the soldering iron to separate the batteries &amp; use the multimeter to verify the voltage of each battery before, during, &amp; after charging each battery separately.

It is possible to charge all of your batteries at the same time using parallel connectivity, but I suggest you test &amp; charge your batteries individually until your get your battery pack operational again--then you can get a 48V charger to simplify future charging.

Best regards!