I would be careful in adding new angels to an established setup. The older angels have probably established territories and will defend them against intruders. If you want to add more angels I would suggest completely rearranging the tank just before releasing the new ones. That way no one has had the time to establish a territory. I would also try to add angels about the size of the existing ones. Once the new ones are released feed them all a bit. Feeding distracts them. BTW, 3-4 years are adults but not old yet.
Charlie