Re: What villains are left for a 4th film?
They can use movies 4 & 5 to set Unicron up as the major enemy for 6. They can start 4 with a tease of Unicron. He sends a harbinger like Sixshot (who would kick lots of ass on screen), like Silver Surfer and Galactus. They're defeated, and Unicron moving slowly has the surviving minions bring back Megatron, Soundwave (or Starscream), and Shockwave's corpses. Movie 4 ends with him turning them into Galvatron, Cyclonus, and Scourge. Movie 5 is them vs the Autobots. Movie 5 ends with Galvatron defeated (still alive), but Unicron looming in the background. Movie 6 can then just lead off as a collosal battle for Earth, and eventually have the Autobots and Decepticons team up.
I would like to the Dinobots, and Predicons introduced in 4. Kup, Ultra Magnus, and Hotrod in 5 (minus an Optimus death since they already went there in 2).
Once you eliminate the impossible, whatever remains, no matter how improbable, must be the truth.