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: DOFP-10(2)Dawn Of The Planet Of The Apes-9.5(3)Guardians Of The Galaxy-9.5(4)Captain America: TWS-9(5)Edge Of Tomorrow-9(6)How To Train Your Dragon 2-9(7)WOWS-8.5(8)Godzilla-8(9)Neighbours-8(10)Amazing Spider-Man 2-7.5(11)Lego Movie-7.5(12)Transformers: Age Of Extinction-7.5(13)Robocop-7.5(14)Sin City: ADTDF-7(15)300:ROAE-7(16)The Equalizer-7(17)Million Ways To Die In The West-7(18)47 Ronin-6(19)Monuments Men-5(20)Ride Along-5(21)I, Frankenstein-3