While Transformers 3 did quite well in the box office, that didn't translate to sales of Hasbro's, the company that manufactures the action figures the movie's are based on line of toys, mainly due to the same characters being used in all three movies. So what is their strategy to boost sales? Bring in an entirely new cast of robots in disguise for the fourth (and some say final) installment. There was talk for some time that Shia LeBeouf, Rosie Huntington-Whitely, Josh Duhamel,and Tyrese Gibson would not return, meaning an entirely new cast, but this is the first time there has being talk of new Autobots and Decepticons. We still don't know whether these will be entirely new creations, or old members of the property's vast cast of characters that were not revived for the previous movies.
Creating new characters just to sell toys is nothing new. Hasbro has already done it, killing off Optimus Prime in the animated movie from the 1980's to push new character Rodimus Prime (a move that had fans in uproar, so much so that Hasbro had to reverse it pretty soon after). In one way, it does eliminate the nostalgia factor that was a big part of the trilogy's success, but it will give Michael Bay free rein to do what he wants with these characters without fear of fan reprisal (of which there has quite a bit of already).
Source: TFW 2005 and Bleeding Cool