"...how can I say it?" is better for your first question.
1, 2, and 4 are good.
For number 3, if you say "Why aren't you quiet?", you are asking the person a question. The person is making a noise and you want to know why.
This could be an indirect way of asking someone to be quiet - Imagine a teacher tells his class of children to be quiet. 5 minutes later, they start talking. So he says angrily, "What aren't you quiet?" ...and the children stop talking.
"Why don't you be quiet" is correct.
Student or Learner