A comparison of Optimus’ face from the first movie and the fourth. I chose opposite expressions, too. He’s smiling in the top one.
AOE has him looking angrier and a bit older in a way, like Sentinel. You can almost see how the vibrancy has been bled out of him over the eons, but even more so with what he went through between the first Transformers movie and AOE.
War ages everyone, and leadership is heavy.