Here is my uptake on teh 4th Villain:
Carnage- Not Enough Time to do him since supposedly there is 10 minutes of Venom.
Chameleon- Doesnt Tie into the Story and No way possible to fit in
Electro- To Big of Story to do in a movie with 3 villains already.
HobGoblin- Either Way were getting a Goblin that is Harry in the story, Im not worried over if its GG2 or HobGoblin
Hydro-Man- I could See Hydro-Man working with the Sandman story so im not cancelling this highly doubful possibility.
Kraven the Hunter- Easy to do on screen but what would his point in this story be? I could see him hunting the Lizard in future movies but not this one.
Jackal- Very doubtful
Lizard- Already have the actor and We have a way to do him, but i think a villain with his calibur should wait for a movie of his own. (but not going to cancel him out yet)
Mysterio- Easy to do, dont need much backstory, dont really care for the character but he can be done easily as a small villain. Not going to cancell him out either.
Rhino- Could easily be done IMO and could fit with the story and still be a easy small villain, highly plausible.
Scorpion- IMO he is a big enough character that he could have his own movie with him, but i could see him as a small bank-robber or something but I doubt we'll see him.
Vulture- I could see him sharing a movie with Scorpion, but IMO seeing this character on screen might seem rather cheesey.
Shocker- He could fit easily too with the story IMO he could be done wonderfully also.