I would definately seperate them and nix the dead/dying colony.This will save anything else of possible infection/sting etc and at the very least help your tanks over all health durring this time. I would NOT suggest a FW dip, but, dip your remaining 3 heads in clean container of tank water saturated with iodine (according to the directions) for the recommended time(also usually in the directions) IIRC its something to the affect of a minute .Ideally follow with some fresh carbon, this will also help what ever may continue to be floating around.Time and sanitation is of the essence, good luck.
-Justin