4 yr old not potty trained?

So I babysit a kid who is 4 and he is still not potty trained.. Is this normal? I know all kids learn at different times but he is a smart brave child! I feel that the parents just aren't focusing enough on him. They are a beautiful family!! I just don't know how to address this.. Changing a diaper on a 4 yr old is a bit ridiculous to me.. Thoughts? I'm definitely not bashing the parents!! Although I have just started out with children so maybe I just don't know enough. 

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