The Xback's will have a higher aggression level next to RTG, so it might be better selecting reds which are larger in size. Are you throwing all 4 fish in at the same time? Otherwise, one might be territorial.
4 aro com is still risky. Even at 5, my friend still managed to lose 1 aro to his mean RTG. Luckily, it was a banjar red.
How big is the tank?