It just sounds illogical. Seems like the four-blade, since it moves less air than the others, would need the shroud more. It might have been that the later cars that had the five-blade fans also had a shroud. That doesn't mean that they had to have it, only that it was there. In any case, a shroud will help with low-speed situations because they direct airflow through the core instead of just moving air around under the hood.