Actually, I find his knowledge of martial arts to be very much in character for him specifically. Remember that Four's history is that he was the heir to an empire. In many governments with an imperial or monarch as ruler throughout history, the nobility is expected to know how to fight. Nobility, especially the upper echelons, were expected to be able to effectively wage war if needed. This still holds true today around the world, even if we in the U.S. pay it little more than lip service with the President also being the Commander-in-Chief.
Now, if Four had been some random guy of Asian descent that was a master martial artist, then I could get behind your point, However, Four had been trained as a warrior by the best warrior in his father's service in expectation of rising to the throne of Emperor (before Step-Mommy-Not-So-Dearest intervened) and had participated in military campaigns before his frame-up. Given this, I don't see him knowing martial arts as any kind of stereotype. He knows martial arts because he was heavily trained in it, and that training was part and parcel to his backstory as a character.