Well, it's a matter of opinion on that, but from everything I've read / heard / seen , a Red Tail Black Shark will always be territorial against it's own kind, hence they will thrive better on their own. The reason you will have had success with a shoal of 3 or 4 is because their is too many of them within one community and they are forced to get along because of this.
Four RTBS's fighting would seclude to nothing, which is why they won't fight.
Anyhow, it is each to their own - which is what makes fishkeeping so appealing in the first place.