Last week at Florida Supercon in Miami, Gregg Berger, the G1 and Fall of Cybertron voice of Grimlock, had a Q&A session with the fans in attendance. Not surprisingly, someone asked him if he was asked to voice Grimlock in Transformers 4 if he would. After a couple of humerous comments on how Grimlock would've shortened the previous movies by the use of his tail as a weapon, he said something quite interesting:
"I will bring justice to the character, you'll see."
That definitely sounds like he's voicing Grimlock. Take this as only a rumor for now though considering there is no footage of this (it was just conveyed by word of mouth). Thanks to Citricon for the info.