Having had many UTIs myself, I understand your daughter's pain. There is nothing more miserable--from my perspective. Little girls are more susceptible than boys, but there are things that you can do to help avoid these infections. Some of the previous responses address several of them.
1. Limit baths--especially bubble baths.
2. Watch her diet. Certain foods, caffeine, etc will trigger infections.
3. Be sure that she is drinking enough water during the day.
4. At 4, busy day care workers don't always have the time to help wipe tender bottoms. Alert your daycare worker to the problems, and ask for their help. If your little one isn't wiping well, this could be the culprit. Offer her instruction on the right way to wipe.
5. Do not use any scented or colored toilet paper, baby wipes, powders, lotions etc in the area.
6. Watch what soap you use and where. Wash her genital area with clean water only.
7. Try changing the detergent/fabric softener you use on her underwear. A product with no fragrances or dyes is best, ex All free and clear.
8. Make sure her underwear is all cotton.
9. Try to encourage her to keep her hands clean and away from the area. Not always easy at this age!
10. Consider tracking her infections. . . is there a common denominator? A food perhaps? or a visit to grandma's house? This might help you narrow down some of the culprits.
11. Definitely sleep without panties.
BTW, my children were unable to use the disposable type diapers and pull ups due to skin sensitivities. You might even try switching brands to see if that might make a difference.