The cop did the correct thing. I feel as solid about this as I did when I witnessed a black MP blow away a white soldier who had been threatening his wife and would not put down his weapon when ordered to do so. Both, imo, were human acts untouched by 'racism.'
A firearm is much different.
Most people are too inexperienced to even be able to kill with a knife if they wanted to.
It is not that easy.
And regardless of the knife attack, less lethal means could have been tried, like a warning shot, or a single shot.
Why 4 shots?
That allowed no possibility of survival.