DMC4 isn't that bad. DMC2 is the series' black sheep... and rightfully so. 4 just doesn't live up to 1 and 3, but it won't ruin your memories. Same gameplay, but you're stuck with Nero half the time and, well, he sucks. You also have to do the same stages twice as a result.
As for the story, I can't remember what the fuck happened in that game. :P But again, who cares?