The Cannes Film Festival is on this year from May 17 to 28, to promote new and upcoming movies to media and reviewers, and two Transformers billboards have been seen there, to make sure that the movie is in the minds of the media, despite it being too early to screen the fifth movie there.
Unless of course, they are going to show more footage like the 20-30 minute IMAX footage, and have on hand some Transformers Movie stars and the Director there to promote it to the media.