I personally found 400NL to be one of the easiest transitions I ever made. If you can beat 200NL for 6ptbb for 40k hands, then you can do the same with 400NL.

I've always taken shots with 15-20 buyins for the next level whenever I'm running hot and feeling confident - and I depend on poker to pay the bills. If you get your ass kicked, moving back down doesn't suck like people think it will. Once you get past the ego hit, you'll find your old games feel so small and easy even if there isn't really a big difference in skill between the 2 levels.

I reccomend moving up and giving it a shot. In my opinion, the money you bring in from 400NL is where it all changes from poker being a profitable hobby to something much greater.