[Mr. Orange]

1) 5.5BB/100 at 2/4 is not sustainable. It was gonna come down eventually. 2BB/100 is pretty solid. You should be happy with that. That being said....

2)I'm confused. If you've lost $800 playing 2/4 how is your winrate 2.0BB/100???

3) If the whole difference between your "profit v. a loss" as you say, is one hand where you got rivered, then you haven't played enough hands. Which begs the question, how many hands is this sample size?

4) To answer your original question. -200BB downswing is pretty rough but they happen. There has been some debate about this topic but the general consensus seems to be that -300BB is the biggest downswing that would be caused by variance.

Truthfully, without more info, it sounds like you got extremely lucky at 2/4 and moved up way too soon.