I assume theyve kept it at level 4 because the first act is aimed at up to that level - after all not everyone does all the quests, kills all the groups of monsters etc so not everyone will necessarily hit level 5 by the end of Act 1. Though you'd have to skip a fair bit I would have thought not to make it to 5.
So they need to know if people can finish it using level 4 characters.
