My 4yo has recurring yellow discharge from her vagina. she took keflex last month which cleared it up, now its back. why does this keep happening?

There are several possibilities. Having had a recent infection and the antibiotic, it could now be a yeast infection. She needs professional attention ASAP -- take her to the pediatrician right away. Do not attempt to treat it without first having in-person medical evaluation. Good luck.

Recurrent discharge often means retained foreign material in the vaginal folds/vagina. It could be toilet tissue/fecal matter or irritation created by sitting in soapy bathwater. Direct exam is needed to sort this out.