TF3 spoilers below, so do not continue if you haven't seen TF3.
(This post is complete speculation and not a rumor or fact.)
Transformers 4, 5 and 6 will happen and TF4 will not be a reboot. This was confirmed by producer Don Murphy. The only questions are: Will Bay, Shia and the rest of the cast come back? Will half of the cast come back? Will the trilogy be continued with brand new characters? Megatron, Starscream, Soundwave and Shockwave are all dead and I can't see Unicron in live action. There has to be a main villian in TF4, so who could it be? I have come to a conclusion of who it should be (and don't laugh at me): Igor. Igor should be the main villian in TF4. His master has been killed and he wants his revenge on Optimus. He finds some great power (something like the All Spark or Matrix) and grows to the size of the normal transformers (kind of like Frenzy in TF1). On the last page of the TF3 comic, Igor is shown with a sad look on his face and says, "Master?" At the bottom it says 'The end?' and not 'The end." Yes, I know, TF comics have not been very accurate to the movies, but what if Bay told the writer to put that in there? So, that's my opinion. I would also include Thundercracker and Skywarp to go along with Igor. Who do you think it should be?