Michael Bay's history with the Transformers series has always been flirtatious. Every sequel brings a new round of news stories about how Bay doesn't want to direct; how he's done with the series, yadayada. Then, like clockwork, he ends up signing on to direct, no doubt with a bump in pay after his public hemming and hawing. It happened with Revenge of the Fallen, Dark of the Moon and the currently untitled Transformers 4.
The latter hasn't even gone in front of cameras yet (he's currently filming the crime comedy Pain and Gain), but Bay is once again insisting it will be he is last. He may not be playing a PR game this time, though, as he's informed the LA Times this new film is being designed to pass the torch to "the next guy," whomever that may be.
The quote comes from a piece dedicated to Transformers: The Ride - 3D at Universal Studios, so unfortunately it doesn't bring with it a ton of detail, but Bay did also tell the outlet that his last film in the franchise will feature an entirely new cast (which we already knew), as well as "some redesign of the robots." Feel free to speculate what exactly that will entail, just keep in mind that the last time a Bay-involved property decided to do some "redesign," the Teenage Mutant Ninja Turtles ended up having alien origins.