For your boy's teeth problem, I don't know what to do. However, on your message found you are very unhappy and frustrated.
He is only 4 years old; don't expect he can correct himself easily. 乖 doesn't only equal to he following whatever you teach him.
Try to remember when we were a kid, how many times our parents said we were not 乖!
Even for adults, we are not easy to correct our bad habit.
My 4.5 years old boy like thumbing, he knew it is not good because he hides himself under the blanket. I found he did thumbing when he is very tried and cannot fall asleep.
Besides, he like to tear his 手指甲when it’s long. Thus, the 手指頭 bleeding. He said he don’t like it long. Though I ask him to ask me to cut for him; he tears it when he finds it’s long. I can only cut his 指甲more frequent.
Your boy's case may not same as mine but try to find out.