David Koepp, the writer for the first and second films, also wrote a draft for this movie. You may remember the script for the third movie was written by director Sam Raimi and was mostly a disaster.
Raimi is still avoiding the director’s chair for now, saying he’s waiting for a script to decide. As of right now he’s not contracted to a fourth Spidey. Others are saying he’s also patiently watching the feud between Peter Jackson and New Line to see if he’ll be brought in to direct The Hobbit under Jackson’s producer wing.
The article says the current plan is to only include two villains, and not try to work in a third like the last movie. The current rumor is that the two villains will be Lizard and Carnage, according to an inside source for FreezeDriedMovies.
In the comics, Dr. Curt Connors experiments with regenerating his missing arm and instead turns into the Lizard (I know what that’s about). In Spider-Man 2 and 3, Dylan Baker plays the doctor (he’s the one that explains the symbiote to Peter Parker). It’s likely that we’ll see his transformation in the fourth movie.
Carnage is a creature spawned from combining serial killer Cletus Kasady (who we haven’t seen yet in any of the films) and the alien symbiote that attached itself to Spider-Man and Venom.
There is also a wild idea that Black Cat might make an appearance, but who knows. It’s not like they aren’t planning on a fifth movie. I’ve read as many as six from interview transcripts.