coral beauty, tend towards territoriality, BUT if they are the last fish put in the tank after any other fish they are fine with almost everything, except conspecifics (other dwarf angels). adding fish after the coral beauty can lead to harassment of the new additions.
and no, I myself, wouldn't get a fourth firefish. I don't think it will help the situation at all. you would only be increasing the chances for discord, if two are males and vie for the same female, or if three are male and go after the lone female. or vice versa. and you also thus reduce the amount of available territory per fish. stick with three and see how it goes. you may get lucky or you may have to find another home for the "odd man out" (or woman).