Sounds like your batteries might be on the way out. You might also look at your charger to see if it's charing all 4 batteries. I'm unfamiliar with that charger, but I believe it only has one light. It could have 4 separate charging circuits, so it could charge 3 and one might not make a connection and never charge. This could end up with the light turning green since it only thinks 3 are there. Now your car may or may not run on the batteries with one weak one in there.

Someone correct me if I'm wrong, I don't have one of those chargers.