I think you're seeing something that isn't there. DMC4 Dante was the least mature of them all, especially given his age.
DMC3 Dante was immature, but he matured at the end of the game. And not as the result of "just plain fun;" that was probably his most significant mission ever, as he had to not only stop Vergil, but also Arkham from literally creating Hell on earth. DMC1 was probably the most mature and serious of them all, whose motivation was not revenge and money, it was his end goal in life of just avenging his mother's death and stopping the ultimate evil that his father fought. In DMC2, I don't know, he was there to do a job and help Lucia, and although he seemed somewhat apathetic, he at least didn't crack jokes and he did what he had to do.
DMC4 on the other hand, was like a vacation for Dante, "just plain fun" if you will. He was careless, sloppy, apathetic to the point of psychopathy, and he didn't take it seriously at all. He didn't really seem to care about the Yamato or about falling right into a trap that gave Sanctus what he wanted, and he certainly didn't care about disrupting the entire society of the island. He was a mentor to Nero in the same way that Johnny Knoxville is a mentor to teenagers across America.
Sure, eventually he helped stop Sanctus, but completing a job doesn't mean being mature about it. He just acted like a child through the entire thing really.
If DMC4 Dante is your estimation of maturity, I think you need to change your definition a little.
Edit: I agree with you about Nero being like a younger Dante, although I think his presence was totally unnecessary, and his appearance was even offensive. They should have just used a time machine scenario and got it over with.