The shoe size of a four-year-old girl varies depending on how fast she is developing, but the average is around a kid's size 10. Four is also the last age at which boys' feet are about the same size as girls' feet.
A girl's feet generally keep growing until around age fifteen, at which point she will reach her lifelong shoe size. In most cases, girls usually grow about one and a half shoe sizes each year. When a girl is three, her shoe size is typically around a kid's size 9 or 9 1/2, and by the age of five usually reaches between a kid's 11 or 12.