Great Beginnings in Long beach is AWESOME. We live in Rossmoor too and it's a bit of a drive (about 10-15 minutes depending on traffic usually) but totally worth it. The kids learn so much about animals, life cycles and basic science type stuff. My 4 year old learned (and remembered) which animals have 4 stomachs and how both bats and whales use echolocation to navigate! They don't do work on letters or numbers which seems odd at first, but the amount of "concepts" the kids absorb and work through in one year is amazing.
You do have to get on a waiting list but you may luck out since your son is older. I found out about the school late and only got our son on the list when he was 3 1/2 (some people sign their kids up at birth). We were fortunate and they were able to take him this past summer. Occasionally people who are on the list for so long have their kids go other places, and when and if a slot does open up they decide they don't want to pull their child out of the other program. You can do full day programs, 5 days a week, do half days or alternate days.
So Great Beginnings would be my #1 pick but there are also a few preschools at the elementary schools here in Rossmoor. At Hopkinson and Weaver I think?