What age should I take my daughter to the dentist? She 9months now and has four teeth.

Between 1-3 years. Go whenever you can find a pediatric or family dentist who will see her at her age. Do watch and clean those teeth gently. Make sure she doesn't sleep with a bottle. The pediatrician can sometimes paint teeth with Fluoride to protect them if you can't get a dentist to see her. Vitamin + Fluoride drops can be used if needed. Even though she'll lose the baby teeth, they're still important for now!
As soon as you can. I believe that parents should take their children to the dentist before 2 years old for maintenance exams. This allows you and your child to develop good habits for your teeth hygiene. It also allows your child to see the dentist's office as a friendly place instead of coming the first time when their teeth hurt and scary, painful instruments are being used on them.
Now. See a pediatric dental specialist for the same reasons you see a pediatrician. Any dentist can tell you horror stories of parents who waited. See dentist before there are any problems for exam, cleaning instructions, tooth and jaw development assessment. Diet suggestions, Fluoride management, and so much more. Remember the permanent teeth started to develop before your baby was born. Go now.