MLive has revealed the plot for Transformers 4 in the form of an official press release and also confirmed that it will indeed film in Detroit, Michigan. The plot summary does have some spoilers of course, which give away the basic story of what will be happening in the movie. It's nothing too huge though and most likely you will hear this summary eventually anyway since many future articles will probably use it. If you do not want to see it, stop reading immediately.
As humanity picks up the pieces, following the conclusion of Transformers: Dark of the Moon, Autobots and Decepticons have all but vanished from the face of the planet. However, a group of powerful, ingenious businessman and scientists attempt to learn from past Transformer incursions and push the boundaries of technology beyond what they can control – all while an ancient, powerful Transformer menace sets Earth in his crosshairs. The epic adventure and battle between good and evil, freedom and enslavement ensues.
The Transformers have mostly left the the planet? Scientists have made a big mistake with technology? An ancient, powerful Transformer? That's some crazy stuff! My guess is that the US once again decided to exile the Autobots after everything that happened in Dark of the Moon and some scientists are trying to make technology that could prevent any further attacks by the Decepticons. One could also argue that this ancient, powerful menace could be Unicron.
More minor news from the article is that Transformers 4 will film in Detroit this spring. This means that filming will probably take place there in May and June before moving onto Chicago for four months. There was also early speculation filming could take place in China and London as well. According to MLive, TF4 was awarded an incentive of $20 million on $82 million of projected in-state expenditures from Michigan.