Naps

When she can manage 4h for the first WT, then she's down to a 4/6 schedule. Shift bedtime earlier if she has early morning waking. With brain maturity, she will eventually let you know when she's ready for a 5/5 schedule (when you see an undertired nap that is between 40-55m long), and gradually a first WT that stays at 6h for quite a long while.