Why not consider taking a full shot at 5nl? You can afford to lose 14 buy-ins (see below) and still be properly rolled for 2nl.

Problem with taking a 4 BI stop-loss shot is that you are sick of being stuck at 2nl and don't want to go back there. So, it'd be easy to play scared with the 4 BIs you're risking. Also:

I) you run bad: it's probably common to have a 4 BI downswing
II) you play bad: you're not giving yourself any room for mistakes! You should be allowed - and are probably expected - to make mistakes when moving up, but if you aren't going to spend any appreciable length of time there, how can you expect to learn from them?

Spenda posted an article here a few weeks ago that probably answers a lot of your questions:

http://www.flopturnriver.com/phpBB2/...ou-t81658.html