When I was 4, my father would just strap a life vest on me and let me go.
Waders wouldn't have mattered much as I was just going to end up face down in the water anyway.
I don't remember when I first got my son waders, but they were the 3Forks brand from Cabelas.
It's a good thing that they didn't cost much cuz it didn't take long for him to out-grow them.
Me.... age 4