You can try and jump the batteries to start the charger. I perosnally use a 12v car jumper and place it over 2 batteries. That should be enough to allow the charger to start.

To removed the bottom 2 batteries you need to remove the top 2 batteries, then unbolt the plate that holds to 2 top batteries. The bolt is in between the mast channels. There should only be 2 bolts, then that plate can come out giving you acess to the bottom batteries.
If the batteries are more than 2 years old i'd recommend replacing all 4.