We had a similar experience here, about mid-year. Isaac ADORED his preschool. He was grouped academically with the four year-olds as a young three year-old, and was blowing through all THEIR work, but socially it was just perfect for him. His favorite playmates, as far as I can tell, were two fellow three year-olds... one boy who didn't speak much English but shared his love of zooming cars around for hours on end, and one girl with whom he loved to "play princess," whatever that means. From what I gathered, he played the part of the baby; I'm not sure how that fits into the princess game.
We had kind of clued his teacher in to the fact that he was pretty close to reading, and that he could add and subtract, but didn't push much because he was having so much fun. Then, one day, she stopped me and did the whole your-kid's-really-smart-never-had-one-like-him song and dance. I just kind of nodded and smiled.
We'll see how he stacks up next year in 4K. We're moving from a very low-SES area back to a college town, so things might look different there. However, he really enjoyed his classmates, socially. He loves playing burying cars in the dirt with the five year-old next door. One the one hand, I'm so happy for him when he reads a book about space, but I'm equally happy that he still seems quite happy rolling in the dirt.
I worry about how much information to pass along to his 4K teacher. I worry about him losing his passion for reading and math because he's not challenged enough. I worry about him losing his childish ridiculousness too early because he's (or we are) overly academically focused. It's all a big huge question mark.
OK, I turned your post into a ME-ME-ME! post, but my original INTENTION was to say "right there with ya."
So, yeah... I'm right there with ya.