Hi M.!
Welcome to Westerville!
We moved here a little over a year ago and love the area. My 3yo son is enrolled in a 2 morning a week preschool at Central College and we are very happy there. We have no point of comparison (since he's our oldest) but there is a lot of communication from the teachers (at least once a week) and plenty of opportunitites to participate in school (in the classroom or with other initiatives).
When I looked around last year, I also visited Our Day in downtown Westerville and All the Children on Freeman Road. I ultimately chose Central College because they have a long history, the classrooms they use for preschool are intended for this purpose and they were closest to our home.
There is a preschool fair coming up at the Westerville Community Center (which is a great facility if you haven't checked it out yet- http://www.westerville.org/Default.aspx?tabid=111) and that may help you decide if you can wait. Were you hoping to enroll your son ASAP? I assume so since he's 4. I surfed for a link about the fair but only found this info from an e-mail I sent to some friends: Friday January 9 from 9:30- 12:30. I'm afraid that's likely before you move here.
There was a preschool fair at Grace Brethren Church last January. Let me see if I can find out it there's another one this year.... Well no luck on a Google search so I wonder if that fair has moved to the community center and that's why it isn't on the internet anywhere. Sorry this isn't much help...
Your realtor should be able to give you a website or phone number of an agency that helps pair up families with preschools. They were at the fair I went to last year and I can't for the life of me remember their name but it seems like something a realtor should have access to for relocation purposes.
There are great family resources in Westerville which is why we moved here. If you have any questions or want input on anything (even kid-friendly restaurants or a good pizza place) feel free to contact me!
HTH-
J.