Hard to say, I would think it will depend on your cats, but generally speaking, I would think you might be better of getting and older female. Do your present cats get along well? They might not appreciate an intruder. Could you possible get two kittens, so they have each other to play with while the other two hate them?
Well, I disagree - older females are going to be preyed on by the younger male and seen as a threat by the other female, where another (young) male is more likely to become friends with the other male (though both will eventually terrorize the female(s) as they all get older (in my experience).