Is my baby ready to learn to self settle?

My midwife told me, and I've read in many places, that babies don't have the brain development to self settle until they're 9 months old. My daughter is almost 4 months now, and she's just started falling asleep by herself if I put her in her cot and leave her for a bit. She is a great sleeper, but she has always been a nightmare to settle, so obviously I'd be stoked if she could settle herself now. The only thing is, she's only settling herself some of the time - like once or twice per day. So my question is, should I start deliberately trying to teach her to self settle all the time, or is she still too young?