Hi K.,
At that age, you need to pick your battles! Is it so horible to let your 4 year old pick out & dress herself? I let my daughter pick out her outfits every day except Sunday (for church) and if we're going somewhere special where she needs to look nice, and even then, I let her choose between 2 or 3 outfits that I've chosen. One day we went to a playdate at the park and she wore her tie-dyed shirt, some pink shorts and her halloween socks. Plus, she wanted her christmas hair bows. I figured it was ok. She was just expressing her own "style" and her independence. As long as she's clean, fed, polite, and happy, who cares? Soon enough you'll be having other issues! Enjoy your daughter's little quirks while they're still little! Have fun with her! They grow up too fast.
If she's taking too long dressing, then I'd set up some sort of reward system for dressing faster. In our house we have "daddy dollars" (play $ with my husband's picture on it). If you do things that are asked of you in the amount of time asked to do them and with NO whining, you get X amount of daddy dollars. Everything has a set amount. Daddy dollars are turned in for TV time, computer time, friend time, etc. If my daugter wants to save them, I'll give her $1 real dollar for 10 daddy dollars. Likewise, if she misbehaves, etc, I'll take away daddy dollars.
I would also talk to your daughter about how blessed and lucky she is to have a mommy that can buy her things because some mommies can't afford to. If she refuses to wear the things she picked out that you just bought, then take the clothes away and take them (and her) to a homeless shelter and give the clothes to a little girl that doesn't have any extra clothes. We did that with our daughter and some stuffed animals. She now appreciates her things more and has empathy for children who don't have as many luxuries as we do.
Good luck & I hope you find a solution!
Shellie