All 4 were Astros at the same time. You are probably thinking of Joe's older brother Phil, also a knuckleballer who never played for the Astros.
I don't remember the order of the rotation but can you imagine facing Ryan's heater plus the best curve in the history of baseball followed by Niekro's fluttering knuckler. Then Richard's 100+ fastball then Scott whose pitches looked like fastballs until they dropped straight down at the plate.
I think it fell out of his back pocket. He may have tried to throw it away when the ump was looking for it.
