My DS4 will be starting private kindergarten this fall. He has a winter birthday, so this would be early enrollment if he were going public, but the private school cut off is months later and he just makes it. So, although he'll be starting a full year earlier than all the kids I know who are his age (including all his current friends), there will be other 4-year-olds in his class.
It's a small school (100 kids total in grades K-12), with just 10 kids in the kindergarten class, ages 4-6. In the morning, all 10 kids are together for what sounds like a very academic kindergarten. Then, in the afternoon, the 4-year-olds have the option of going home or staying for something more like preschool: naptime, music, art, Spanish, free play. My husband and I both work, so my son will stay for the afternoon session. If all goes well, the following year, he will advance to 1st grade.
I chose the school because it was founded with gifted kids in mind, although you do not have to be gifted to attend. They do extensive pretesting and develop an individualized curriculum for each student in each core subject at every grade. Most kids are working at least one year above grade level--many are working much higher--but all without skipping grades. It's kind of like having a tutor, but in a peer-group setting. The flexibility will be great for my seemingly lopsided son, who currently reads at about a 3rd grade level, but shows little interest in math.
My big concern is that he's never been happier at the play-based preschool he attends 4 full days a week. I'm not sure whether it's because he just recently moved from the 3 year old room to the 4 year old room, or because he's made an extremely close friend, but either way, he's doing absolutely great--so well that I've been wondering during the last week or so whether we made the right decision by enrolling in kindergarten this year. However, I've spent enough time on this forum to know that having two good school options is a good problem to have.
And then there are those days when I still wonder whether it's really that unusual for a young 4-year-old to read at a 3rd grade level. My son just seems so...normal.