S.C.
The concern about four year olds is that the average four year old is not able to grasp reading, nor willing to put in the practice time.
My daughter started at age 4, but I had to beg (YES, BEG) at least 10 different instructors before I could get her in. Then I finally got her in by asking one person to just give her a try for two weeks to see what would happen.
The reason so many people say no is they have all started with 4 year olds and then realized they are not disciplined enough to practice a half hour every day on their own. They cannot read written instructions or work in the work books. It's frustrating for the teacher.
Even though my daughter was a beginning reader, I still had to sit with her for every practice the first year and then read all the lessons to her so she CLEARLY understood them.
SO - if you think either you are committed enough and/or your daughter is committed and well read enough to read through weekly instructions written on a pad of paper. That she will practice every day for at least 30 minutes. That she will have the hand span to reach all the notes. That she will have enough understanding not to get confused when reading left to right. And that she will be able to perform in front of a crowd for the yearly or bi-yearly group performances. THEN you should go ahead.
We did - with my help it worked. Now she is 7 and is still playing.
GOOD LUCK!
S