Hi,
  
Here's an article that offers insight on pre-K:
   
Pre-K (What Exactly Is It)
http://www.education.com/reference/article/pre-k-what-exactly/
   
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.

my son is 4 years old and he just started pre k..