I am one for keeping children back until they turn 5. Particularly if they aren't 5 until mid year. She will be up against children a whole year or more ahead of her in age/stage/maturity. I particularly took into consideration that fact when she will be younger than her peers in high school. She might be 16 1/2 and have 18 year old peers in her grade. I believe the social aspect is more important at this age (4 1/2-5) than the academics. Plus, they are babies for such a short time. Part of the reason I kept my DD back was to enjoy her for that one more year.
Ultimately for us, we wanted to give our child the best leg up we could. The way our schooling is structured in Aus, we did that by giving her the most opportunity to be able to learn and compete better. By keeping her back a year, she was more mature, had learnt more and had grown so much more socially and emotionally in the extra year she was home. She started school this year at 5 1/2 years old. She is now the top of her class, is a leader and actually helps the teacher with explaining things to her peers who don't fully grasp some concepts.