2-3 inches even without good ventilation, especially with only 23w cfls, just make sure the temp isn't above 90.
1-2 inches is best. If a CFL is 4 inches away instead of 1 inch away it is delivering only one-sixteenth the amount of light. If your plants got burned then they had to be touching the bulb, and that indeed is too close.
Hey toasty biz, good to seeya on the posts again. This man was just one of my mentors, listen to him. HE KNOWS HIS SHIT! 2 inches i'd say is best.