A.L.
My daughter will start pre-k in the fall. She will be 4.5 when she starts. We did not want to start her before age 4, but I found it was difficult to find a place I liked that had a slot open for a mid-year start. (Her bday is in Feb.)
I registered her early, in Oct, to start next fall, and I think that helped a lot. Otherwise, if the classes are full there really isn't anything you can do.
We picked the school based on what we liked. We visited 5 total, discounted 2 right away because we didn't like them. Ultimately, we went with the one we liked the best. You'll know which is the right one for your child -- go by your instincts and feelings.
Good luck!