It seems that the studio and director Sam Raimi are at odds over who should be the villain in the film. Here's an excerpt from the sites' article.
Raimi is pushing hard for the Vulture to be the big baddie, something he also pushed for in the third film to star alongside the Sandman but didn't get and we all know how that one turned out. Vulture was to do his evil deeds alongside the new Goblin and Sandman. A single concept art sketch can be seen in "The Art of Spider-Man 3" book. Vulture and Flint Marko would have been cellmates who escaped together, with Vulture pressuring the more passive Marko into committing crimes.
On the flip side, who does the studio want to be the villain? Our source says they seem to only be interested in featuring which ever character is selling books right now but basically they have no idea, just not the Vulture.
The site also reported that many of the department heads working on the film were told of the production halt last Thursday, and also that the film is being referred to as Spider-M4n around the office.
CLICK HERE for the site's full piece and be sure to keep it right here as we'll be sure to update you on any and all news coming fromm Spider-Man 4 (or Spider-M4n) as soon as we have more information.