usually a battery does percolate when charging on a battery charger. not a kmart 3 amp speacial but a real battery charger. that is why your supposed to remove the caps to allow venting when your putting a heavy charge to a battery. you have 4 in succesion that battery was probably had more charge than the others so it was percolating. when they percolate they tend to lose water. so make sure its still ok with water.
you should be ok. if they are deep cycle they are made to be charged alot. i cut my power and once a month i turn it on fire up my gen and run for 15 min or so then i cut power and leave it alone.
i could be mistaken but the charger on board is supposed to sense a full charge and stop charging. if one of your batteries is weaker than the other, is the only thing i can think of that would cause only one to percolate and the charger not to cut off.
if your concerned load test them or check with hydrometer.