While talking to the New York Post, Spider-Man 4 producer Todd Black confirmed Kirsten Dunst's involvement in the upcoming sequel after months of speculation led Sam Raimi to personally ask Dunst to join.. Apparently, it worked.
Black did talk a bit about the villain for Spider-Man 4, first by ending rumors that Morbius the Vampire would appear in the movie, and then saying:
We're just coming up with who the villain's going to be now. We'll be
shooting in New York again. Trust me, people will appreciate who we pick, because it'll be a big part of New York.
Black also mentioned that the villain will be from the comics and not someone invented for the movie. So, if the villain is from the comics and is "a big part of New York," that likely rules out rumored villains Electro and Carnage, Peter Parker's professor-turned-reptile the Lizard, or other villainous possibilities Kraven the Hunter, the Vulture and Man-Wolf.
Could it be the Kingpin, leader of the New York's organized crime, already portrayed by Michael Clarke Duncan in Daredevil? Considering Fox owns the rights to the character, that seems unlikely. With little else to go on, speculation will have to suffice until Sony announces the villains, which may happen once the script is delivered at the end of summer.