By saying there are 4 scum, you are saying that by lynches and NKs alone, the game could be over during Day 3. That's before considering Daykills, which by nature would be more likely to be used on town targets than mafia targets, though you could also argue that the potential for failed NKs or successful hits on Mafia might balance that out. Still, the natural progression says the game would end at Day 3. That's not a good balance point for a game this large. That's the big reason why I don't think it could be 4 scum.
That maximum number should be read that there are not more than 2 Spies. Scum has either 1 or 2 power roles, and spies fill out the remainder.