Would you really say the enemies and bosses of DMC4 can keep up with the complexity (or sheer power) of the combat system? I don't think so. Don't get me wrong, there's a lot of cool things going on with the enemies, but not enough where I can say the combat system is worth its very hefty weight in gold. That diminishes what I consider the greatest difference. I was largely disappointed with the bosses of DmC, but the fodder was more than adequate enough and in this series you'll take good fodder when you can get it. For what it's worth, level design/pacing in DmC is better for the most part. As much as I wouldn't want to do some of those platforming parts again, there are worse things, like the whole infamous repeating the whole game thing. It is a shame both games employ stupid methods of locking difficulty behind a dozen hours of play. I guess you could say I think every DMC outside the first is rather flawed.
I'm not sure that's a fair standard to apply, necessarily. That's a big part of why the Style system exists in these games: to help extend the (measurable) skill curve well past the point where the enemies cease to be a problem. It's definitely true that enemy design/enemy responsiveness to your moveset is an important half of the equation, but the point of basically EVERY well-made character action game is that some players will be challenged by the enemies and some players will work on finding increasingly stylish ways to whomp the enemies without ever taking damage.
I'll echo Dahbomb's criticisms of the DmC fodder enemies (though I *did* like the shield-generating witch enemies apart from the color-coding thing and the teleporting dual-sword-wielding demons). Enemies ought to be capable of defending themselves, but they shouldn't be totally immune to certain weapons.
Part of this, though, is why I increasingly think that DMC games need to be less linear. Bayonetta's got certain challenges in it (namely Rodin and the final Angel portal) that are purely optional and also vastly above the rest of the game in terms of their challenge level. It'd be great for DMC5 to have optional content in it that seriously tests the mettle of even the best players without ever feeling unfair.
I *do* think that DMC4 suffers for a bunch of the fodder enemies clearly being designed around Nero's moveset rather than Dante's, but when it really all comes down to i-frames, hitstun, and juggle, it's not like Dante's out of place in any sense - he's got many more tools to work with and some of them are just slightly further out of reach.