Please let your daughter start expressing herself through her choice of dress. Four years old, as anyone who has ever interacted with a four-year-old knows, is plenty old enough to have opinions, personality, and preferences of one’s own. Clearly, your daughter is beginning to express what she thinks fits her.
Is it a phase? Who knows! Maybe! Maybe she will develop a more masculine style of dress as she gets older. But maybe not, maybe this will be something she does temporarily. Maybe she’ll go on to dress "like a boy" and "like a girl" for the rest of her life! How well-rounded would she be then?
Regardless, what does it matter how she dresses? What does it matter how any of us dresses? One of the amazing things about having this life is that we have these bodies that we get to adorn however we like, and how we choose to show off our bodies tends to be visual clues to other people like us about who we are and what kind of things we’re interested in.
And your daughter, at the wise young age of four, is already asserting her individuality and her own tastes by being interested in boy’s clothes. What’s the harm in letting her do so? You will be telling her that you care about her thoughts and opinions, that you trust her to make her own decisions about her body and herself. These are great things to empower kids with, regardless of their age.
If you’re uncomfortable with it, ask yourself why. Why does she have to dress "like a girl," in pink? Why do you think it would be harder for her if she dressed with a more boyish style? Perhaps you have some underlying homophobia going on. That’s something to take a good hard look at in yourself. Check out the stories by Ivan E. Coyote about her nephew, who, as the stories go, "likes to wear dresses." One of them is on her CD You’re A Nation. They’re moving and sweet, and a great example of what it’s like to listen to what kids want, and how we can honor them and encourage them to express themselves.
Good luck, Sandra. I hope you can ease up the gender policing of your four-year-old and let her be whatever she’s going to be, creating a safe space for her to express herself as much as possible.