As a mom of 3 teen age boys and a coach for a team of 90 kids, I have a viewpoint. It disagrees with some of the other posters, but here goes. Ages 3 and 4 are way too young for organized sports!
Ages 5 and under should be learning to play, have fun, learning to listen and learn about the world thru activity. Young children should not be learning about competition and "beating the other team."
Gym and tumble programs, kids movement programs, swim lessons are best.
In my 19 years as a parent and 7 years as a coach, I have seen the competitiveness among kids (and their PARENTS) get downright dirty and nasty.
I saw a referee have to jump in and stop an argument between 2 parents this summer over which kid took 1st in a swim race. The kids were 7 years old.
I know of a football player who started taking designer "muscle enhancers" at the age of 15 b/c he wanted to be as strong as the 17-18 year olds.
One of my neighbors has a daughter in karate & they started her at 3 so she could be the "youngest black belt in Texas." The parents wanted this, obviously not the 3 year old. She is now 9, has an eating disorder and serious sleep/ anxiety issues that have caused her to miss much school. The youngest black belt in TX will have to repeat 3rd grade, while her friends move to 4th.
This doesn't happen to everyone, of course. But it has a tendency to occur in families who place a lot of emphasis on their kids' organized sports. And sometimes it starts at a very early age. Sometimes not. For many, it never happens at all. I just advise keeping things in perspective.
I would warn everyone against starting kids down this journey too soon.
Let your kids have fun: go for hikes and bike rides and teach them to be water safe by age 5. Or get them good swim lessons. Teach them to throw a ball, and kick a ball in the street or at the playground. 3 and 4 year olds should run around the park with other kids.
Have a good time. The competition starts soon enough.