I don't think that it was racially insensitive writing nearly as much as it was lazy writing. Four is every non-protagonist samurai from every samurai movie ever made. He's an unkind efficient killer who has been wronged and seeks revenge. He does nothing but practice his swordsmanship, meditate and brood silently. If they build a strong story based on his extreme stereotype then fine, but based on what we've seen so far I'd be willing to bet we're going to get something very stereotypically asian. Seppuku or talks about honor should be coming some time soon.

All that being said, he's still probably my favorite crew member. I love that before he knew his past he was a badass cold blooded killer who is going to kill everyone eventually (except the crew of the ship), and after he learned his past he became... a badass cold blooded killer who is going to kill everyone eventually (except the crew of the ship) to reclaim the throne of Feudal Japan. It made me really happy when they were locked in the cargo hold and running out of air-- in his delirium he finally let loose a smile. Hooray!

Little aside response to OP: I agree totally that One is insufferable. I disliked his character and his dialogue so much that it effected my opinion on the other members of the crew, and the show overall. He was just the fucking worst, except during the episode where he had a different actor and where he played the real Jace.

As far as bad episodes there's really only one that irked me deeply: the sexbot episode. I had no problem with a double agent android storyline, but then the episode centered around Android's hurt feelings and jealousy. Cmon what a drag.