The director outlined the setup for the Mark Wahlberg-starring movie for TMZ.
"The movie is going to continue four years from the attack on Chicago, which was in the last movie," he said.
"It's going to have the same lineage, but it's going in a full new direction. It actually feels very natural how it's going in that direction... We're doing a lot of things that are really going to make it fresh and feel new."
Bay praised Pain and Gain collaborator Wahlberg, who he said had joined the cast following an original, untrue rumour that he was already in the running.
Regarding his disputes with former stars Shia LaBeouf and Megan Fox, the director said that "fame is very hard on young people".
The fourth Transformers movie is due to start filming in spring 2013 for a 2014 release, while Pain and Gain will open worldwide on April 26, 2013.