A comment found buried in an article that looked at the Transformers Movies, and how well it is doing in non-American markets, makes mention of there being four Transformers movies after The Last Knight, currently on the books from Paramount... but be aware that this is a comment from an analyst who isn't from Paramount, so it could be just speculation, or he has came across official documentation during his review of the franchise.

We already knew that Hasbro/Paramount had planned to have three movies in quick succession from TF5 to TF7, but if this info is true, that would be another two movies after TF7... which could also be each following year, to give Hasbro that annual boost to toy sales that they enjoyed during the 6 month period around the first four movies.