Here's a particularly relevant excerpt:
"All pre-K programs have three characteristics in common. They are (1) governed by high program standards, (2) serve 4-year-olds or sometimes both 3- and 4-year–olds, and (3) focus on school readiness."
I also recommend that you contact the pre-K program(s) in your area to find out their age requirements for enrollment.
Below are some additional resources that you may find helpful for this stage of your child's development and education.