Noob question with Antigravity Batteries.

With the 4 cell antigravity battery, should I be purchasing the 6-volt charger or the 16 volt charger?

4 Cell Antigravity Battery:
https://shop.antigravitybatteries.com/products/starter-batteries/small-case/ag-401/

6 Volt Charger:
https://shop.antigravitybatteries.com/products/accessories/battery/chargers/antigravity-batteries-6-volt-charger/

16 Volt Charger
https://shop.antigravitybatteries.com/products/accessories/battery/chargers/antigravity-batteries-16-volt-charger/

12V. Voltage is listed in the lower left on the face of the battery.