It really depends on how the individual school "defines" it. Your best bet is to call/visit the schools that you're thinking about, and ask how their curriculum/days compare...
In my area, preschool for 3-yr-olds is half-day, Tuesday and Thursday. Preschool for 4-yr-olds is half-day, Monday & Wednesday & Friday. Prekindergarten is for kids that might be 5-yr-old, but not ready for the "rigors" of a full time kindergarten program yet...
It's a chance for them to have another year to "get ready for kindergarten" and to develop better social and "basic academic" knowledge....
Bottom line, at least in my area, is that preschool is for 4-yr-olds, pre-K has older children who need more time to mature before kindergarten.
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