Agree with Jack's guidlines, I play a lot of $11 and $22 SNGs and that's generally how I operate. I tend to start to loosen up when the blinds hit $50/$100 (guideline thanks to Drarfman's vids). Even when 3-4 people bust out early due to overly ambitious pushes, I still stay relatively tight, the size of the pot makes it generally not worth trying to exploit marginal edges.

I also make it a point to pay careful attention to my table image, and try not to be too obvious when I'm stealing the blinds. You've obviously got to bluff more when you're 4 handed, but I try to be hard to get a read on. The tight table image early in the SNG helps when you start to grab pots when you open up. A slow-building sense of aggression works well for me and keeps my opps guessing. A good read on the aggression level of your remaining opps is crucial too. You have to be able to sense when you are skating the edge of their tolerance with your steals, and be able to fold a few semi-decent hands to cool them off. I suppose quite a bit of my success vs bubble-outs is due to how well I am paying attention to the tone of the table. Take what they will give you.

In SB vs BB play, I'll fold about 50%, complete the blind about 30%, and raise about 20%. When I just complete, I'll usually fold to a raise since BB has position. But every now and then I'll sneak a strong hand in there and play back. When I raise, it will usually be 4-5x BB again due to position. All of this of course can be adjusted depending on how your neighbor wants to act.