My son tried it when he was 4 yrs, but he didn't care for it. It just wasn't his thing. There were plenty of other 4 yr olds out there that were having a great time, though. If she is "into" it, then by all means let her have at it. Just be sure and talk with the person(s) who will be coaching her and make sure that what you envision for your daughter (fun fun fun, not competition competition and win win win) is the same as the coach's, and try to get her on a team that is primarily also 4 yr olds. It isn't as much fun for the little ones if their team is dominated by older kids who are faster and more skilled (part of the reason I don't think my son cared for it.. his team - of 5- had one MUCH older player who ALWAYS was able to run, steal, score, etc before anybody else got a chance, and my son just tuned out).
If she is excited to play, let her go for it.