My son turned 4 in August, so I completely understand your concern. Obviously, being his mother, I also think he is extremely smart and probably could handle the work of kindergarten come this fall. In fact, our neighbor is a kindergarten teacher and she swears that he would probably be one of the brightest in the class (even though he would also be one of the youngest). We are going to wait until he turns 6 before we send him. It really comes down to a few key things:
1. We want him to always have the advantage. By being younger than everyone else by a year or more, he is all ready at a disadvantage.
2. While my son is tall for his age, he is fairly skinny. He has a friend who weighs almost 20 lbs. more than he does (and is only 3 months older). What does the above average, in size, child who was born a year earlier than by son look like?
3. My son has a fairly large interest in sports. Understandably, this may subside as his schooling goes on, but if it doesn't, he will always be one of the youngest on the field. Keeping him back would give him an advantage in middle and high school that those shoved ahead would not have.
4. Finally, I think it comes down to maturity. A LOT can change within a year. I look at some of the boys he is friends with who are still 3, and my son is leap years beyond them. That also means that the boys who have turned 5 by now are probably leap years ahead of my son. It is important for him to be able to build relationships with the boys in his class, and that will be harder if those boys are more mature emotionally and socially.
This is what helped make our choices easier...I hope it helps you a bit too!