Well, sure you can get extreme cases @cazzie. Especially in cases of abuse, or simple ignorance. I was talking about the average 4 year old.
I once saw an ER show about this child and his failure to thrive. He was five, but only as big as a 3 year old, and could barely walk. He had other issues. The doc started asking questions and found out that “all” the child would eat was oatmeal and milk. The parents let themselves be bullied into feeding him only that, because of his temper tantrums. For 5 years he’d had nothing but oatmeal and milk.
The parents loved him, they weren’t being abusive, they were just ignorant. They had to get steely eyed and firm, under the threat of losing their son to the state.
They said it was hell, but they stuck with it. Six months later he was rapidly catching up.