I guess it depends on on how old she will be when school starts - will she be 5, almost 5, or still in the middle of being 4? How mature is she? Has she done any preschool before? Will she be bored in 4 year old preschool or overwhelmed by pre-K? Where I live, preschool is for 3 and 4 year olds. There is a Pre-K, also called Junior Kindergarten, for kids who could start kindergarten but have late birthdays (our cut-off is Dec.1, and pre-K is for b-days between Sept and Dec, as well as other qualifying factors, like being relatively immature, less socialized, etc.). Preschool is either 3 or 4 mornings a week, while Pre-K is 5 mornings a week.
My daughter's b-day is August, so we don't get a choice. She is in preschool 4 mornings a week now, she'll be 5 this August, and she will be expected to start kindergarten the following month. I chose 4 mornings as opposed to 3 because she loves going and that 1 extra day that she gets to go and is out of the house is worth it for her, because she is so happy there. At this point we don't have any reason to think that she won't do well, or that we should hold her back from starting kindergarten this Sept.