My daughter started a few months shy of turning 4. My son started shortly after turning 3. Every child is different. In my son's case, when he was around 2.5 I told the school if they got an opening he was ready. They normally wait till they are 3 but he was very ready! He didn't end up starting until the summer session when he was about 3 years old and a few months. Both of them took to preschool very differently. My daughter although older, cried when I left. My son hardly looked up to say goodbye! This past week however he hasn't wanted to go, kids do this so be prepared.
To send her or not is really up to you, like I said, all kids are different. You are right, she has years of school ahead of her, but if you have a great preschool, it's a place for her to socialize with peers her age, play play play, learn many things like; waiting your turn, caring for a friend who just got hurt, doing activities that you may have never thought of to do at home, manners, hygeine, etc.. things that can be taught at home of course but it's just different when Mom/Dad aren't around. Sometimes I go to my son's school and just watch him w/out him knowing. I'm constantly amazed at what I see. I also volunteer at the school a fair amount, know all of the teachers very well and just overall, feel very comfortable with my kids in their care - that helps. The only regret I had was with my daughter, I only sent her two days a week so I think that is part of the reason why it was hard for her, she just wasn't there enough. I started my son out at 3 days a week and I think that is just perfect. For him of course, I miss him terribly, but while the kids are gone, I do all the cleaning/shopping/cooking so that when they are home, I'm not running around, I can just be with them. Sorry, I got a bit long winded here...
Best wishes whatever you decide,
M.