First of all, your provider has worked hard and she is a caregiver or childcare provider. Considering how well she has done, it's rude to call her a babysitter. She probably feels that way too even if she is too polite to say so. Give her some respect and stop thinking of yanking her income when she's done nothing but perform well.
Secondly, you have said it yourself. She is fine right where she is. Let her mature the rest of the way in the great place she already is. To be doing all the wonderful things she is doing at her age, you could leave her with this provider until first grade even. I'll be providing Kindergarten for 2 of my daycare boys this fall & winter and mostly likely a 3rd girl will get through the curriculum in that time too.
Relax. Children need a little more time to be young before the highly structured life begins.