Oh great, 4 batteries to deal with it.
Every so often on a weekend or Federal holiday when I was home and public transportation did not run I would check my batteries fluid, top them off and use a small 12 volt charger to top them off. I guess this could be considered balancing them. I hear the automatic wheelchair battery chargers only go to like 90% before they cut off and never reach the float status or the one that boils the fluid for a little in all cells.
Id have to look em over to get some ideas. I thought if I had to get another vehicle Id get the hub motor bicycle wheels and those at 1000 watts at 48 volts can do 45 mph.
The bounder is another idea as it looks like it uses go cart parts.