In comparison to Devil May Cry 2 (which did add some good stuff to the series, admittedly), Devil May Cry 4 was not a mistake by any means. I think it could have used a good bit more work, but overall it was fun and interesting.
It did enough good things to keep it from being a bad game. It introduced Nero, who was a fun character to use (as well as one with a lot of potential, both gameplay-wise and story-wise). His Devil Bringer was a welcome addition to the game, so much so in fact that controlling Dante in Stage 12 left me a tad disoriented without that delightful arm.
It also gave Dante the ability to switch between styles on the fly, which led to some fantastic combos and an unmistakable sense of variety. The additions to each style were appreciated (though Royal Guard still blows), as well as the addition of a "new" style for Dante, however un-diverse it may have been. I didn't like that both of Dante's new weapons (I don't consider Gilgamesh completely new) were silly and useless, and I especially didn't like that Dante didn't end up wielding Sparda at some point, but that is forgiveable. At least his Devil Trigger was vastly superior to that of DMC 3.
The upgrade to the graphics was pretty nice as well. It was also comforting that enemy designs in number 4 were vastly superior to those of number 3, which consisted of enemies with scythes and enemies of piss-poor design, and no exceptions (except for most of the bosses).
In spite of its shortcomings, I don't consider number 4 a failure. I don't consider it much of a success, but I wouldn't even consider it a failure in the least.