You might be able to do a few things. A shunt inside the controller is often modified to increase the amp limit. This usually leads to better performance on hills. Some controllers can be able to handle 12 more volts, but often controllers are made to be used in the 36-48v range, so it might go poof if you try more volts on a 48v controller. Opening the controller and looking at the capacitors can give you some clues. If the can like things inside are labeled 63v, then it won't take more volts from a 5th battery.
So in general, the best way is to replace the controller with a higher voltage one, and add a 5th battery. One of the problems with performance with scooter type bikes is they are a bit heavy. So bicycles with similar motors perform much better. Another option that will help, would be replacing sla batteries with lifepo4. The weight will really decrease, and there will be a lot less voltage sag, so the ride performance will be more like the first mile on sla's.
But there is no magic wand that makes a 500 watt motor perform like a 2000 watt motor, so real performance increases are only going to come with a bigger bike. It is what it is, a moped, and it will never become a motorcycle. I don't know what type of increase you are looking for, but a 5 mph increase in speed should be possible with more voltage. With scooters, replacing the controller can be a pain though, since on some , all kinds of wiring goes through them for everything on the bike. Hill performance gets better with the shunt mod, so I'd be temped to try that first if it was my bike.
THE LIPO RULES. NEVER ABOVE 4.3V NEVER BELOW 2.7V DON'T PUNCTURE
Ideal charging /discharging range for Lipo, 3.65v minimum 4.1v maximum
See battery technology section, FAQ thread at the top of the page for lipo noob info.