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.
2014 movie ratings out of 10:
1)X-Men: Days Of Future Past-10(2)Dawn Of The Planet Of The Apes-9.5(3)Captain America: The Winter Soldier-9(4)Edge Of Tomorrow-9(5)How To Train Your Dragon 2-9(6)The Wolf Of Wall Street-8.5(7)Godzilla-8(8)Neighbours-8(9)The Amazing Spider-Man 2-7.5(10)The Lego Movie-7.5(11)Transformers: Age Of Extinction-7.5(12)Robocop-7.5(13)300:Rise Of An Empire-7(14)A Million Ways To Die In The West-7(15)47 Ronin-6(16)The Monuments Men-5(17)Ride Along-5(18)I, Frankenstein-3