As noted, the difficulty achievements are indeed not stackable. However, you don't have to play through the Mission 4 times to get the four Difficulty Achievements.
When you defeat the Dark Apprentice, you'll get a Cut Scene and your Achievement will pop. Directly after this, you can pause the game and Exit Game. When at the main menu, you can Continue and you'll start the Fight with the Dark Apprentice again. Pause the Game, go to Options, and increase the Difficulty. I did this after my Play through of Sith Lord. I increased the Difficulty to Sith Master, fought the Apprentice and the Achievement popped for Sith Master. I think this will help a few people who want to get these achievements easier and quicker. This may also want to be put in the OP if possible. :P