The competitiveness of the sport really depends on your coach. So far, we've had only really laid back "let's learn the game and have fun" coaches. This fall's soccer coach, we will see. She was taking about winning yesterday and that bothered me. They don't keep score at the U6 level. I don't really like all this winning loosing talk. I think at the youngest ages, it should be about having fun, learning the game and making friends. Although, baseball is considered more competitive than soccer around here, we had such an awesome set of coaches. They were all about let's be a team and learn to love baseball. Our head coach was a dream coach. Any kid starts talking about winning or being the best and he was all over that.
I know four seems really young to start playing soccer (or other organized sports), but for the homeschooled or stay at home preschooler it is a blessing. Lovemycrew is right. By the time kids are 10 or 12, the other kids are very competitive on their own. They don't want to play with a kid who doesn't know the game. The coaches are all like, "Ugh, who wants to take this kid who is ten and doesn't know the game?"
At the t-ball/U6 level, the kids don't really have that much focus. Keeping them heading to the right goal or not throwing water at each other in the dugout is a major challenge, but most of the kids seem to really have fun. Sometimes, it's nice for kids to learn how to pay attention, teammanship, and good manners outside the home/church/school setting. My dd is a sports nut. She loves sports, not because of winning or being the best, but because she makes a lot of friends and has fun. For her sports are such a blessing, they give her a way to fit in when that is so hard for her.
Girl Scouts will do that for her too, but there is more time for conversation in scouts and that can be the kiss of death for her as far as making friends is concerned. She really needs that structure.