At Transfomers 4, the latest appearance of the leader of the Autobots, Optimus Prime appears in the form of custom-built trucks typical Western Star, a brand under Daimler Trucks North America.
It's a much more sleek, less boxy design for the leader of the Autobots, but all the trademark design elements are still there. The truck is a completely custom version from Western Star (a subsidiary of Daimler Trucks North America).