Who the fourth villian is depends on how the filmmakers intend to use him or her. What do they need this villian to accomplish in SM3? Does he or she only need to provide Spider-man with a quick fight, to show off SM's heroic nature? Or, is this newest villian being set up as a future threat in subsequent movies? Then there is the chance that the filmmakers want to bring back a previous villian. So let us narrow down the options.
If the 4th villian is only a cameo (such as an intro fight scene), it would most likely be a b-list villan such as Black Cat, Mysterio, Rhino, Scorpion, Shocker, or Vulture. I rule out Black Cat because PP/ SM already has enough love interests in this movie. Mysterio's powers may require too much background to be understood by a casual fan. Also, his effects maybe too expensive for only one scene. Rhino might look silly if he is only an actor in a costume. The Thing and Juggernaught come to mind here. If he is CG, he may be too expensive. Scorpion would fall victim to the cost of the CG effects needed for is tail/stinger. Vulture would require the same CG wing effects that Angel needed in X-Men 3. My guess would be Shocker. His powers wouldn't need much explaination to casual fans. CG shock waves would be less expensive than to animate an entire character or body part (such as Rhino, Scorpion, and Vulture).
If this lastest villian is seen at the end, suggesting further development in a possible fourth movie, I only see three choices: Carnage, Electro, and Lizard. These are the only characters I can see being able carry a movie themselves. They are the only villians I could see the filmmakers being able to write an entire plotline for. Also they provide alot of eye candy to the audience ( both comic fans and casual fans).If it is true that Venom appears for only one fight, this might imply that he survives SM3. That would rule out Carnage, since to me, he shares too many similarities to Venom. A future movie with Spider-Man and two clones seems uninspired and unattractive to the filmmakers. Electro is possible, but he would come out of nowhere to casual movie fans. They would be left scrathing their heads, wondering who he was and how he belonged in SM3. The Lizard is the only one who has some existing movie development (as Dr. Connors), and can fit into SM3's plotline. The Lizard seems to be the best guess.
Finally, if the filmmakers want to bring back a villian, that leaves only two choices: Norman Osborne/Green Goblin and Doc Ock. At the end of SM1, NO/GG was as dead as dead can be. A physical comeback maybe too fantastic for the casual audience to believe. And a ghostly appearance has already been alluded to in SM2. A Green Goblin announcement at Comic Con would underwelm the comic fans. However, if they announced Doc Ock at Comic Con, that would create "madness". Also, many have wondered if he really died at the end of SM2. Here, Doc Ock seems to be the best choice.
There you have it. Depending on how the fourth villian is used, my guesses are Shocker, Lizard, or Doc Ock. What do you think? Is any of this valid or reasonable?