First of all, Slash Film had an interview with Steven Spielberg and he was asked if he thought Bay would come back for another movie:
"I hope so, because I think he made the best of the three with this last one.
I certainly can’t imagine anybody other than Michael being equipped to make
another Transformers. He’s invented a genre and he’s got the secret
Michael Bay had an interview with Channel News Asia while in Singapore for the opening of Transformers The Ride (which opened a day early yesterday) and was asked if he would return and his reply was:
"How did I know someone’s going to ask me this question today? We’re talking
about it, but there’s nothing right now. I’m going to do a tiny movie first,
then we’ll talk about it, if I potentially do it."
This comment makes me about 80% sure Bay will return to direct TF4. Just the fact that he changed his mind from an absolute "no" to considering it shows that he does have a lot of interest in the franchise. My prediction: Michael Bay will direct Transformers 4.