You can put down the pitchforks and torches. Despite Transformers 4 introducing a new cast of robots (and humans), Peter Cullen will be back as Optimus Prime. The Huffington Post was talking with Michael Bay about his bodybuilder movie Pain and Gain, and he revealed that Cullen will be back in part 4:

I spoke to Peter Cullen at Comic Con earlier this year. He said he doesn’t know if he’s going to be back as the voice of Optimus Prime or not. He’ll be back, right?
Of course! Of course.

Before the first “Transformers,” you got some heat for the whole “flames on Optimus” thing.
Yeah, I know.

But if you got rid of Peter Cullen, people would be extremely upset.
Listen, when I heard his voice, it gave me chills.

The last quote is interesting as they were dumb enough to make Peter Cullen audition for the voice role he created. The studio originally wanted Clooney for Prime, but fans would’ve rioted if they didn’t have the real Optimus in the movie.