Re: What villains are left for a 4th film?
There are plenty they could go with, my personal choice would be Thunderwing with a close adaptation of Stormbringer being the basis for the movie. We could see the likes of Jazz, Megatron etc in flashbacks to keep it all familiar.
Other than him there is Galvatron, The Quintessons, Straxus, Nova/Nemesis Prime (although a 3rd fallen Prime on the run might be a bit much) or there is also Sixshot, who would be a pretty formidable enemy.
2015 movie ratings out of 10:
1)Kingsman: The Secret Service-9(2)Ex Machina-8.5