Update: Michael Bay has confirmed he is returning to direct Transformers 4 with a release date of June 29th, 2014.
"We’ll be there in the summer of 2014," he confirmed, as well as addressing (albeit vaguely) whether Michael Bay will be returning as director or not. "Michael had a debate because he sort of felt at first that he'd done three. Michael and I are both the same in that we always like to top ourselves. That doesn't necessarily mean bigger. It can be bigger in places, it just means different, but we're going to push really hard as fresh and new as we can be."
He compares doing a fourth "Transformers" movie to what they're doing with the "G.I. Joe" sequel by having a few characters return as well as telling a story that does follow-up from a plot set-up in its predecessor without necessarily fully "rebooting." "Again, we're going to try to do a hybrid there where there will be some characters that come forward--we think, we're still in the process of figuring it out--and some characters that don't, but it will definitely be a different story."