A 4-year-old is old enough to know what they are and aren't allowed to grab, in terms of common household items.
> However, if you constantly yell at four years old, four years old will learn to ignore everything except yelling.
If you consistently show a four-year-old that only yelling will be followed by physical intervention, then they will learn to ignore everything except yelling. If you consistently show a four-year-old that a quiet "I'm going to count to three... one... TWO..." will be followed by physical intervention, then they will learn to pay attention to that.
The default attitude of most four-year-olds is "make me" and if you let them know what will result in you making them do the thing, they will pay attention.
Not that I never yelled at kids. I did. But this reaction on that situation sound like that.
So, he was probably wrong in how he handled it, but it was quite heartfelt what he said, he was handling it as well as he could, and perhaps that was badly. It was hard for me to second guess him under the circumstance.
Four years old can be taught to cut soft vegetables under close supervision. However, if you constantly yell at four years old, four years old will learn to ignore everything except yelling.
Not that occasional yelling harms that kid or something. But, yelling often is more of adult emotional reaction, not a rational reaction to real acute danger.