Are you honestly saying that a 4 year old has the judgement to be out playing unsupervised? I'm sure glad that none of my neighbors share your feelings.
Honestly our neighborhood is very, very close. It's like a constant block party. The kids play, the moms talk, the dads throw frisbees, footballs, whatever, everyone throws balls to the dog's. The kids are being kids, we aren't scripting or directing their play. I have to disagree that they play differently when parents aren't around. When my kids are playing in the back yard, unsupervised, they do the exact same things they do when we are out there chatting.
I am out there to watch for teenagers driving too fast on my side stree, to make sure they have their helmets on, to make sure they don't wander too far away. I'm out there to watch for suspicious cars. While you may think that is absurd, we have had several reports of 2 different people trying to abduct children in the area. It may be a slim chance, but it's not one I'm going to take.