If their water levels are maintained (with distilled water only), golf cart batteries can be cycled to 50% charge and back up to 100% typically 200-250 times.  Depending on how much you've used them, it is possible that you have used them that much in four years.  At this point, since they aren't holding a charge, I'm willing to bet that your batteries are toast and cannot be revived.

The other factor that often gets overlooked is what temperature they are stored in when you're not camping.  Most battery chargers, including the one in your Pod, are not temperature sensitive, meaning that they give the battery the same charge regardless of the temperature they are in.  Over long periods of time, this can lead to the batteries being either undercharged, which leads to sulfation and kills the battery, or overcharged, which can boil off the water in the batteries and expose the lead plates to the air, which also kills the battery.

That's why I only use temperature sensitive battery maintainers at home.  My personal favorite is the Battery Minder #1500, but there are several good ones out there.