Hi I.,
I'm a dental hygienist with a 2.5 yr old and a 7 month old. I applaud you for being concerned and knowing the importance of brushing now, even if there are only 4 teeth! We went through stages with my daughter--I remember when we could only get the brush in there for a few seconds, too! The important thing is to try to make brushing fun so that it doesn't become a power struggle, and to be consistant. If you find that you can only get the brush in there for a few seconds, then do that 3 or 4 times a day instead of once. Always start with brushing your own teeth and let your daughter see how much FUN it is (haha). Let her try to brush your teeth, then her own, when she's at that age. Set the rule that Mommy always brushes baby's teeth first, then it's baby's turn. If she does nothing but chew on the toothbrush, she's still removing plaque. Try singing a song to go along with it, be silly and fun (even if you feel dumb!) and eventually she'll start getting into it. Also, you need to work on her saying AH. You can make the brush a train, or whatever, just like the tricks you use to get food in her mouth! She might like sitting on the bathroom counter and watching herself as you brush.
Also, don't be discouraged if you hear that you're supposed to brush her teeth for 2 minutes--that's the standard for adults with 32 teeth! Just brush as long as she'll let you and work on it every day. Even 10 seconds (or 2-3 if that's all you can get now!) is better than nothing. Brushing before bed is the most important time, since the bacteria will be brewing in her closed mouth all night.
Remember, the key to preventing decay is not just brushing 1-2 times a day--you need to limit juice or other sweet drinks, sticky sweet foods like rasins or candy as snacks, and definitely don't let her fall asleep with a bottle (or sippy cup!) of milk or juice--this is the number one cause of the front teeth decaying out in babies.
I went from a baby who resisted brushing, to a toddler who brushes and FLOSSES every night with me! And she loves it! It's part of her bedtime routine, and it's fun for her. If you want to start flossing (if her teeth are tight together), get one of those kids floss holders so she won't bite your fingers. Probably a little young to start now, since brushing is still a struggle, but consider it in the future. The younger you start, the easier it will be later.
I hope this has helped. Good luck, and don't give up! It will get better!
B.