Things
A lot of people look at games as more than just one super-awesome aspect. DMC4 did have lots of gameplay potential, but overall, it was weaker than its predecessor, and just a glorified HD version of DMC3's gameplay with some slight tweaks. The fact that the new character had less skills to work with, and Dante's retreaded journey really took the game down a little bit in a lot of people's eyes. DMC3 remains the pinnalce DMC experience to date because it had everything that made a great DMC game, and just a great game. DMC4 just sorta tried to do the same thing, but with less effort.
Saying "DMC4 was rubbish" was surprising to me too, and it's still his opinion, but he's not entirely wrong. It's a point where you have to look a bit more objectively at things. I wouldn't say it was rubbish, but it wasn't the best.