Transformers 5 & 6 not yet guaranteed
Despite the billions of dollars that Paramount has made so far with the first three Transformers Movies worldwide, and a fourth movie that is almost guaranteed to make a profit due to all the similar factors in play, they are not committing to a full second trilogy until after the box-office figures come in for number 4.
It seems like an odd thing for them to announce, unless it is standard procedure for such an expensive investment. (committing to 3 movies of this budget could cost them a fair bit if they tried backing out due to a flop for number 4 or 5)