Authority can come from respect, understanding and even love. My Father (who never hit me) had authority over me even when I was big enough to easily resist any physical force he might bring to bear - it was because of the respect and love I had for him, not because of any fear of force. on the other hand, His father constantly hit him, and still had no authority over him, particularly in those older years.
To use violence, especially in a measured, mechanical, humiliating way, shows that you believe that might makes right. And you believe that your 4 year old does not own his own body, which he does despite his age. And shows that you don't know how or are unwilling to earn even your own sons respect, preferring the blind obedience of a dog.
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