I don't think a Spider-Man 4 will be necessary, but my picks:
1. Black Cat-a little sexual tension is always good. My pick: Halle Berry, it can be a chance to redeem herself for Catwoman...Just joking.
2. Lizard-He's already been set up in the first two films, and I heard he might be in the third, so give him a shot.
3. Scorpion-always liked him, and I would love to see how they would do his suit on the big screen.
4. Rhino-reasons for seeing him same as Scorpion.
5. Electro-Ditto.
6. Mysterio-I think he will be in Spider-Man 3, or maybe that is a rumor. Anyway, I think he would also be a good villian.
7. Vulture-Ditto.
Like a previous poster suggested, I would put Morbius in the Blade TV show/future feature film, and let Tombstone take on the Punisher.