Here is a wiring diagram showing how to wire the three batteries.
As long as the fourth battery is properly secured to the scooter and the wire's insulation is protected from rubbing off then it should be safe. Wire sleeving and rubber grommets can be used to help protect the wires. The wires can be soldered to the batteries to prevent them from falling off while riding.
Gregg hill
I just got my new batteries and all that stuff. The problem I ran into is only three batteries would fit in my equal smart metro so I have a 36 V system with three batteries and I need to wear them in series. I really need somebody to either walk me through this or make a diagram with the connectors black red and so forth and make it idiot proof because this is my first time even trying this please help.