I haven't read all of the previous posts, but honestly Mel, in my opinion, I woudn't. I won't be sending Lily to school until 2012. She will be 5 going on 6. It's not that I don't think that she will be ready from most aspects of development, because I do, but to me that one year is worth it for her to be ahead of her peers and to be the same age as them, if a fair bit older (between on average 3-8 mths older than most of them, but up to a whole year, which would be the situation for you with A, he would be just 4yrs, and some of his peers would be 5 1/2yrs or more). She will most likley do better academically than her peers in most areas of development this way and will not have the possible issue of lagging behind as she gets older, kwim? I know I am going to go absolutely bonkers with her here for a whole more year, but imo, it's worth it for her in the future, not just in the younger years, but also in high school and even later life. I will be sending her to pre school for 2-3 days a week the year before and next year she will go into day care for 1 day, plus we go to playgroup 1-2 days a week and she has 3 children here who we see weekly that are her age, plus some younger and some older.
I'm hoping it will help her have better self esteem, think of herself as intelligent (even though she is anyway!), and just generally have better self esteem. My brother went to school when he was four, had to stay back the next year and has always lagged behind since then because he though (and still does think) he's dumb when he's actually very intelligent. It was just a stigma he was left with when he was so young, my eldest brother tortures him about it still and it has stuck with him to this day and he's 40.

ETA, ok just read back and I'm thinking of propper big school, like some others, 5 days a week, uniforms and all......but 3 days at (what would be like NSW pre schools) I would do it.