Regardless of start age, most preschools and pre-K programs require the child to be potty trained before enrollment (or far along in toilet learning development). The Preschool Readiness info center has potty training articles, as well as an offer for a free potty training kit from Pampers.
I would also suggest that you review the expert and parent answers that other parents with the same or similar questions received in JustAsk:
Lastly, contact the preschools and pre-K programs directly to find out the enrollment policies and age requirements specific to each school. My experience when enrolling our daughter in preschool was that there was a lot of variance across preschools or not the same start date across all schools I called.
I'm also including a resource to support your family with your child's current developmental and learning stage.