[Related Link: TRANSFORMERS 4: Is Nemesis Prime the Newly Revealed Freightliner Vehicle?]
Getting straight to the point, this theory that the new 'freightliner' vehicle possibly being the Decepticon version of Optimus Prime has a little more evidence behind it. While on location in the middle-of-nowhere—Adrian, Michigan—a fan filmed a scene with Optimus looking like he's being chased by the 'over-the-cab freightliner' who could be Nemesis.
See this in the first video below. Then in the second video, the fan filmed the Transformers 4 director doing what he does best... creating explosions. All of which were around Optimus.