Reshooting has been taking place in the last couple days in America.
Either Bay has had some new ideas and wants to include them, or he's found something missing in making the plot work and needs to bridge a couple scenes with this new scene.
After doing some reshoots already a couple months ago in China, this is getting a bit late to be changing or fixing parts of the movie like this.


Also, an entertainment magazine has an article on the TF4 movie, with some plot details... so beware of spoilers.