What he said above.... plus I would trip any breakers next to the battieries - just in case.
Keep in mind the age of the old and new battieries as well.
They like to be the same type and size but also AGE.
IMO 6 mo old + new would be fine. One year old + new = maybe?? Two year old + new = I would not do .... get 4 new ones or wait a couple more years and replace all at the same time. Or put 100 watt of solar on your shed/garage and use the old ones for that system?
Something to think about.
2017 Axis / 25.4: Big Foot Levelers, Aims 1200 Watt inverter, Ceramic Window Tint, Full River AGM 6V batteries, Front Hellwig Sway bar / Rear Track Arm, Safe T Plus Steering Stabilizer, PD 4655 converter, Hard Wired Surge Guard, WEboost + WiFi Ranger, LED Headlights/Driving lights, TPMS, Surge tank, Tornado flush, Viair Compressor