J.M.
My 4 year old had the same problem. Here's how we fixed it.
1. No more baths. Showers only. Sorry. The soap and dirt in the tub is irritating for some little girls. And don't use liquid soap - it's too strong and is harder to wash off than bar soap.
2. Have her pull down her pants all the way to her ankles when she goes pee, and have her sit with her legs as wide as possible. Then have her finish peeing and sit there for the count of three.
3. Have her sit with her legs wide and wipe sitting down, from front to back.
4. If she looks red and irritated, put a little bit of aquaphor on her vulva after the bath - externally only!
It took about a week and a half of this for my daughter's issue to resolve. Good luck!