UPDATE: Bay himself has spoken out about directing TF4, and though the studios are pushing for his involvement, Bay is still weighing his options:
Right now I’m not decided on Transformers 4… Studio’s are passing rumors but I’m not sure what I’m doing? I know I’m going to do Pain and Gain early spring but that’s it for right now. I’m leaving all my options on the table. I’ve got several Studio meetings in the next two weeks.
What we do know is that there’s already a rough plan in place for the film since producer Don Murphy already came out and said the next film would not be a prequel and that it would continue the franchise’s focus on human characters in an effort to kickstart a second Transformers trilogy. According to Vulture, this remains true and TF4 will “delve deeper into Transformers canon to mine older characters and lore." One of their insiders says the following that we can take as a hint:
”Regular people might not care about bringing out a character like Sentinel Prime. But believe me, fans of the series cared."
Fans who saw Transformers: Dark of the Moon understand that it would be difficult to continue the series as is due to how of the characters ended up by the story’s conclusion. We can therefore expect radical changes and plethora of new characters, perhaps even mining stories from some of the other animated television series.
Expect Transformers 4 to release in summer 2014.
The whole story was found here.