Sam Raimi has stated that the script for Spider-Man 4 should be ready by the summer, so the villains that the wall-crawler will be facing should be confirmed soon after that. I just want to recap suggestions, rumours and fan picks on the villains:
Lizard - Lizard is almost definitly going to be in the film. Sam Raimi and Dylan Baker have both said they would like the character to appear in the upcoming film. Dr. Connors has been in the series since his brief mention in the first film, so it's about time we see him in action.
Carnage - Carnage is a villain a lot of fans want to see. Their logic behind his appearance will be that the sample that Dr. Conners kept during the third movie will be the Carnage symbiote. This could work, especially if Lizard gets involved too. Though this is unlikely to happen. But maybe we'll see him in the upcoming Venom film.
Morbius - This is an interesting pick. Morbius wasn't one of the big villains in the rogue's gallery, but the Living Vampire could be in the film. Sam Raimi himself has said that he could appear. It is an odd choice and could be hard to pull off. But if done well, it could make a good movie.
Mysterio - Quentin Beck was at one point rumoured to appear in Spidey 3, played by Bruce Campbell. That didn't happen, but some fans want it to happen. Mysterio was a good villain, and it would be interesting to see him on the silver screen, but it is only a rumour.
Electro - Raimi said at one point that he would like to intoduce Electro. He is a good villain that could work well, but nothing else has popped up yet.
Vulture - Vulture is a very likely choice. He was originally in the script for Spidey 3, and they approached Ben Kingsley to play him. They already made concept art, so they could easily fit him into the story.
Kraven the Hunter - Grant Curtis (producer) said he would like to see Kraven in a future film, but nothing else has really been said, so this probably won't happen.
Black Cat - Black Cat was considered to appear in both the 2nd and 3rd movies, but was dropped both times. Some fans would like her to appear, so it could happen. But if it did, I don't think she would be the main villain. She would take up more of a side villain role.
Sinister Six - Raimi said at one point that he would like to create the Sinister Six. It could happen if all the villains were made up of previously introduced charaters, but Sandman is the only villain to have survived. So it could happen, but will more likely happen in the 5th or 6th installment.
So that is it for now. More news should arrive soon. My updates will be infrequent, but i'll try to keep up.