I don't agree that they can tell the fighter gets 2 attacks. Attack rolls are a mechanical phenomenon, but not in the fiction, which is just a melee in which some are more puissant than others. (Ranged attacks, especially if ammunition is tracked, break this somewhat - they always have a tendency not to fit with D&D's rather abstract combat system.)They understand "this wizard can cast 4th level spells" or "that fighter gets 2 attacks" just fine. i don't think the distinction is as "under the hood" as you are suggesting.
4th level spells are probably not extricable from the fiction.