My earliest memory when I was 4 years old, and I saw my older sister leave for her first day of 2st grade. It was memorable because I knew in a year I would be going off to kindergarden. My personality was definitely different, I see myself then as a small naiive child that believed elementary school is where all of the "big kids" were.
I've been reading through a lot of the posts, and I find it interesting that most memories (like mine) revolve around the four year old range. I wonder if this is because of the transition from daycare to elementary school that marks a large change in a child's life or simply because this is a threshold for our memories.