The logical first choice is Optimus Prime's 'brother' Ultra Magnus. The Autobot is the white version of Optimus' vehicle mode in the cartoons, comics and toy lines and Ultra Magnus was even named the leader of the Autobots in the animated Transformers The Movie. Even though his reign as Autobot leader was short lived.
Another theory is this could be a Decepticon. Reason being that Bay has released so many vehicle modes of Autobots, it's time for the villains. If this is the case, this could be Motormaster who is the leader of the combiner team the Stunticons. If this is is the case, it remains to be seen if another combiner robot will be in the fourth film since the combiner in Transformers: Revenge of the Fallen was not liked by much of the fan base.
Finally, and this was brought to The Daily SuperHero's attention on Twitter, is that this could be Nemesis Prime. If you are not familiar with Nemesis Prime, as you can see in the above image, he is pretty much the Decepticon version of Optimus. Described as a clone of Optimus who is a "dark mirror" of the Autobot leader. (You can read more about Nemesis Prime on the Transformers Wiki page, by clicking right here.)
Once brought up, Nemesis Prime seriously makes perfect sense. Why? Because Megatron is dead. Who is going to be the main villain anyway? Starscream? Dead. Shockwave? Dead.
[Related Link: TRANSFORMERS 4 Set Videos: Is Optimus Prime Being Chased by... Nemesis Prime?]
Galvatron could still be in the cards or Motormaster (who would be lame as the Decepticon leader, no offense Motormaster). However, having a story line where Optimus must face off against an evil version of himself is very, very intriguing. Of course this is just a theory but one The Daily SuperHero hopes Bay is exploring and adapting as the main villain in Transformers 4.
Stay tuned as Transformers 4 continues to film through the summer months.