I think if you back way, way off, life will be better for all of you. You must, at all costs, avoid catering to her every whim. I mean, there's nothing wrong with preparing meals that your kids like, but bribing her with cookies to take a bite of mac n cheese, or only preparing things she already likes is going to create a monster.
I think it's really common for young kids to go through a phase where they'll hardly eat, especially if it provokes a huge reaction from their parents. If your child is on the small end of the growth charts to begin with, it's enough to make us moms panic! I spoke with our ped about it (my youngest is tiny for her age and is quite picky), and he said that if she's hungry, she'll eat. Children will not actually starve themselves to prove a point, even though it seems like that's what they're doing! Our doctor said to put the meals in front of her that everyone else is eating (if she prefers the chicken without bbq sauce, I'll scrape it off, but that's as far as I go to cater to her). If she eats them, great! If not, oh well. She doesn't get dessert or anything else after dinner if she doesn't eat dinner. Other than that, she gets NO reaction from us about what or how much she eats. Believe it or not, she went through a growth spurt recently, so she is getting enough to eat. (I can't imagine how a tablespoon of chicken and a teaspoon of broccoli is enough to sustain a 4 year old, but... apparently it is!)
Like one of the other moms said, I think it does help to talk to your child about nutrition, in terms she can understand. "We are going to eat chicken today because chicken has protein in it, and that helps us have strong muscles! We're having spinach salad because that has calcium in it, and calcium builds strong bones and teeth." Let her help you make dinner - even preschoolers can help wash lettuce leaves and tear them up, or stir sauce. My daughter is much more likely to eat when she has a stake in making dinner and setting the table.
It's just a phase, and the bigger the deal you make of it, I think the longer it will last. Just be calm about it, let her be involved with making meals as much as you can, and let meal time be a time when your family talks and has an enjoyable time, rather than focusing on what she is doing (or not doing, as the case may be). I think that will help. Good luck!