It seems that it is possible that Transformers 4 may end up filming in Thailand at least according to The Hollywood Reporter. Paramount Pictures was one several studios that participated in the "Inbound Roadshow Familiarization Tour" from the Thailand Film Office that is attempting to try to get Hollywood to film there more often by showing off its variety of location. Transformers trilogy location manager Ilt Jones was one of them to participate in the tour.
This doesn't mean the film will take place an Asian country. Nor does it mean that it even be a primary film location. It just means that the studio looked around (on another's country's dime). Depending on the final script, it could be ideal for a specific goal much how Transformers trilogy filmed overseas (Egypt, Russia, etc.) depending on the needs of the scenes. Considering the great pride that Michael Bay takes in making his movies in the United States (such as using Bethlehem, PA as Shanghai, China), I just don't see production spending much time, if any, in the city for Transformers 4. Considering the ever growing importance of the international market in box office sales, all this could change once someone else takes over the franchise from Bay. Thanks to Wynton for the link.