Re: What villains are left for a 4th film?
I think now is the absolute perfect time for Galvatron and Unicron to make an appearance.
Unicron, floating aimlessly somewhere in deep space, could have been alerted to the presence of Transformers on Earth by the opening of the space bridge in TF3. This caused a huge spatial disburbance which could have been picked up by his long range sensors.
Megatron died at the end of TF3, right? I'd imagine that the Autobots would somehow have ejected his body into space to get rid of it. They tried burying him in the sea at the end of TF1, but that obviously didn't work so space would seem a more permanent solution.
That would set up a perfect scenario where Unicron would come across Megatron's body in his slow journey towards earth, and he could rebuild him into Galvatron for some nefarious purpose. Galvatron would be sent on ahead to engage the Autobots before Unicron's arrival.