Can Battery isolator kill 4 batts at once?

If the isolator shorts out, I could imagine that it could happen. Or possibly the isolator went bad, and one battery of the 4 has a short and it allowed the juice from the other batteries to backflow from the good batteries

It's possible but not likely. What type of isolator? Transistor or mechanical? What do you mean by "Killed"? Did the all end up at the same voltage? Totally dead or just low? Did they recharge when the battery charger was reconnected? It's been my experience that dead batteries are more often the result of an ongoing problem with the charging system. Usually they haven't been getting completely charged for a while and it wasn't noticed until they were actually dead.

Hi,

If they are good old Lead Acid type and the charger has been used full time it it possible that they have sulphated plates and they rely on the electrolyte to give an indication of condition= with little or no real reserve capacity.

If one is looking distorted I would agree with CaptJ that the other three fed the duffer till it **** the bed.

Solid State, diode type, isolators are very reliable unless current above their rated capacity was drawn. The isolator can be checked with a cheap digital volt meter like the red one at Harbor Freight with the diode check circuit. Put the volt meter on the diode symbol, flip it on and check for a reading in each direction from battery input to output of the isolator with the boat batteries disconnected. The forward reading should be 300 or less and the reverse reading should be off the scale. Do this for all four inputs. If they are burned out they should read low values 300 or less in both direction or really high valves in both directions, greater than 1000. Remember the isolator circuit has to be disconnected or the test will not work. Hope this helps.