My sister is a natural 4a. I've rollerset her hair, and she's had it done at a Dominican salon for special events.
I will post the pictures when I get a hold of them, but it gets straight and looks just like my hair when I get a rollerset. The only thing is that it reverts within about 3 days (although my sister sweats up a storm at night so that may be why). Note: she's a natural who presses at least once - twice a month and therefore has some heat damage (straight pieces). I'm not sure how much this helps to get her hair straight.
When I do her hair, we just flat iron the roots. At the Dominican salon, they blow the roots. It turns out nicely!