well as several other members have discussed already, the jury is still out on what constitutes truth and fact. MFK is not the place to discuss epistemology. For a better discussion of "facts" I would recommend a book by Hegel entitled "Phenomenology of the Mind."
The suggestion that pygos should be kept in groups of 4 is as much fact as anything related to fish keeping is. This is a posteriori. Pygocentrus piranhas are characins that live in shoals, like most shoaling characins a group of 4 is considered the minimum number for providing the social environment and behavior for a shoal of fish.
yeah well they're not as exciting as some other fish, but they have their redeeming qualities for people that aren't overly concerned with having feeder genocide in their tanks. Maybe yours were lame because you kept less than four?