It's already been acknowledged that Transformers 4 is going to focus on a new cast of robot characters (in part for marketing purposes) and human players - though, at the least, Optimus Prime is coming back (once again, voiced by Peter Cullen). However, people such as Shia LaBeouf, Rosie Huntington-Whitely, Tyrese Gibson, and Josh Duhamel won't be reprising their roles from previous films, in director Michael Bay's supposedly-final outing with the franchise.
One of the main motivating factors behind making fresh-faced actors like LaBeouf - as well as Whitely and her predecessor, Megan Fox - the leads was to appeal to the core teenage/young adult demographic. However, seeing how TF4 is being sold as a more straight-faced film (matching the tone of the final act in Transformers: Dark of the Moon), it might make sense for the human characters to likewise grow up a bit (literally).
The latest TF4 rumors suggest that won't be the case (shocking, we know), as the new flesh-and-blood leads are indicated to both be under the age of 19, as was the case with LaBeouf and Fox's onscreen counterparts in the first two Transformers movies. To be specific, Spoiler TV reports that Bay and his team are looking for an actress and actor to play "a high school senior and her boyfriend a Texas racing driver," respectively.
It ought to be pointed out that Spoiler TV identified the former of those characters as the "new female lead," while the latter is referenced in a more off-hand manner. That could mean Transformers 4 will depart from literally every other film made by Bay to date, by featuring a female protagonist in a capacity similar to LaBeouf in the original Transformers trilogy. On the other hand, that might be little more than wishful thinking because, well... this is a blockbuster made by Michael Bay.
Human characters - as well as the drama they come attached with - are a necessary cost-cutting measure for the Transformers movies, so you can expect TF4 to have its fair share of them. However, Bay has suggested that part of the film will take place somewhere other than Earth (and not just in flashbacks, a la the prologue to Dark of the Moon). The movie also gets going in the immediate aftermath of the destruction-happy climax to TF3, so it won't have to waste time rehashing the Transformers' backstory in any significant capacity.
That is to say, TF4 won't completely abandon the formula of its predecessors, but it looks to incorporate some new ingredients into the mix to help keep this series from feeling stale (for its fans, anyway). You can color me pleasantly-surprised if the film actually takes some 'risks' and centers around a woman, rather than male lead - though, I'll be downright shocked if the female leads actually proves to be interesting (and more than 'upgraded' eye candy).
Transformers 4 opens in theaters on June 29th, 2014.
Source: Spoiler TV