Jean Lyle Children's Center!
http://jeanlyle.org/
It's in Merriam Park/Mac Groveland and I am so grateful that we found it for my 4-1/2 year old. He went to the 3-day morning program last year, is in the 5-day morning program this year. I have already registered my 7-month-old for the 3-day morning program (which he'll go to when he's 3.) The teachers are truly remarkable people, so connected to the kids, and tirelessly inventive with activities. It's a little scruffy - it's in a church basement (not church affiliated,) and it's a relatively small space - but it's still the most remarkable preschool I've ever spent time in. Check out the website. If your son is going to be 4 next fall, he could either go to the 5-morning 4/5 year old program, or the 3-afternoon (M/Tu/Th) program for 3-5 year olds. It's also among the more affordable preschool options in the area.
Good luck!